Big front-end asset/filestructure refactor

- Extract shared Ember components into a “flarum-common” ember-cli
addon. This can be used by both the forum + admin Ember apps, keeping
things DRY
- Move LESS styles into their own top-level directory and do a similar
thing (extract common styles)
- Add LESS/JS compilation and versioning to PHP (AssetManager)
- Set up admin entry point

(Theoretical) upgrade instructions:
- Delete everything in [app_root]/public
- Set up tooling in forum/admin Ember apps (npm install/update, bower
install/update) and then build them (ember build)
- php artisan vendor:publish
- Upgrade flarum/flarum repo (slight change in a config file)
- If you need to trigger a LESS/JS recompile, delete the .css/.js files
in [app_root]/public/flarum. I set up LiveReload to do this for me when
I change files in less/ or ember/

Todo:
- Start writing admin app!
- Remove bootstrap/font-awesome from repo and instead depend on their
composer packages? Maybe? (Bower is not an option here)

less/forum/composer.less

@@ -0,0 +1,238 @@
+// ------------------------------------
+// Composer
+
+.composer {
+  pointer-events: auto;
+  .box-shadow(0 2px 6px @fl-shadow-color);
+}
+.composer-controls {
+  list-style: none;
+  padding: 0;
+  margin: 0;
+}
+.composer-header {
+  list-style: none;
+  padding: 1px 0;
+  margin: 0;
+
+  & h3 {
+    margin: 0 0 10px;
+    line-height: 1.5em;
+
+    &, & input {
+      color: @fl-body-muted-color;
+      font-size: 16px;
+      font-weight: normal;
+    }
+    & input {
+      background: none;
+      border: 0;
+      padding: 0;
+      height: auto;
+    }
+  }
+}
+.composer-loading {
+  position: absolute;
+  top: 0;
+  left: 0;
+  right: 0;
+  bottom: 0;
+  background: rgba(255, 255, 255, 0.9);
+  opacity: 0;
+  pointer-events: none;
+  border-radius: @border-radius-base @border-radius-base 0 0;
+  .transition(opacity 0.2s);
+
+  &.active {
+    opacity: 1;
+    pointer-events: auto;
+  }
+}
+
+// On phones, show the composer as a fixed overlay that covers the whole
+// screen. The controls are hidden (except for the 'x', which is the back-
+// control), and the avatar hidden.
+@media @phone {
+  .composer-open {
+    overflow: hidden;
+  }
+  .composer {
+    position: fixed;
+    top: 0;
+    bottom: 0;
+    left: 0;
+    right: 0;
+    z-index: @zindex-composer;
+    background: @fl-body-bg;
+    height: 100vh !important;
+    padding-top: @mobile-header-height;
+
+    &:before {
+      content: " ";
+      .toolbar();
+      opacity: 0;
+
+      .visible& {
+        opacity: 1;
+      }
+    }
+  }
+  .composer-content {
+    padding: 15px;
+  }
+  .composer-controls {
+    & li:not(.back-control) {
+      display: none;
+    }
+  }
+  .composer-avatar {
+    display: none;
+  }
+  .composer-body {
+    & h3 input {
+      width: 100% !important;
+    }
+  }
+}
+
+// On larger screens, show the composer as a window at the bottom of the
+// content area. We hide a lot of the content when the composer is minimized.
+@media @tablet, @desktop, @desktop-hd {
+  .composer-container {
+    position: fixed;
+    bottom: 0;
+    left: 0;
+    right: 0;
+    z-index: @zindex-composer;
+    pointer-events: none;
+    .transition(left 0.2s);
+
+    .with-pane & {
+      left: @index-pane-width;
+    }
+  }
+  .composer {
+    margin-left: -20px;
+    margin-right: 180px;
+    border-radius: @border-radius-base @border-radius-base 0 0;
+    background: fade(@fl-body-bg, 95%);
+    transform: translateZ(0); // Fix for Chrome bug where a transparent white background is actually gray
+    position: relative;
+    height: 300px;
+    .transition(~"background 0.2s");
+
+    .index-index & {
+      margin-left: 205px;
+      margin-right: -20px;
+    }
+    &.active, &.fullscreen {
+      background: @fl-body-bg;
+    }
+    &.minimized {
+      height: 50px;
+      cursor: pointer;
+    }
+    &.fullscreen {
+      position: fixed;
+      left: 0;
+      top: 0;
+      bottom: 0;
+      right: 0;
+      margin: 0;
+      height: auto;
+    }
+  }
+  .composer-content {
+    padding: 20px 20px 15px;
+
+    .minimized & {
+      padding: 10px 20px;
+    }
+    .fullscreen & {
+      max-width: 900px;
+      margin: 0 auto;
+      padding: 30px;
+    }
+  }
+  .composer-handle {
+    height: 20px;
+    margin-bottom: -20px;
+    position: relative;
+
+    .minimized &, .fullscreen & {
+      display: none;
+    }
+  }
+  .composer-controls {
+    position: absolute;
+    right: 10px;
+    top: 10px;
+
+    & li {
+      display: inline-block;
+    }
+    .minimized & {
+      top: 7px;
+    }
+  }
+  .fa-minus.minimize {
+    vertical-align: -5px;
+  }
+  .composer-avatar {
+    float: left;
+    .avatar-size(64px);
+
+    .minimized & {
+      display: none;
+    }
+  }
+  .composer-body {
+    margin-left: 90px;
+
+    .minimized & {
+      margin-left: 0;
+    }
+  }
+  .composer-editor {
+    .minimized & {
+      visibility: hidden;
+    }
+  }
+}
+
+
+// ------------------------------------
+// Text Editor
+
+.text-editor {
+  & textarea {
+    border-radius: 0;
+    padding: 0;
+    border: 0;
+    resize: none;
+    color: @fl-body-color;
+    font-size: 14px;
+    line-height: 1.6;
+
+    &, &:focus, &[disabled] {
+      background: none;
+    }
+  }
+}
+.text-editor-controls {
+  margin: 10px 0 0;
+  padding: 0;
+  list-style-type: none;
+  & li {
+    display: inline-block;
+  }
+}
+
+// On phones, since one of the text editor controls will probably be the
+// primary-control, we shouldn't hide it completely when it's "disabled".
+@media @phone {
+  .text-editor-controls {
+    opacity: 0.5;
+  }
+}