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/common/modals.less

@@ -0,0 +1,129 @@
+// ------------------------------------
+// Modals
+
+.modal-backdrop {
+  background-color: @fl-body-primary-color;
+  &.in {
+    opacity: 0.9;
+  }
+}
+.modal-body {
+  background-color: @fl-body-secondary-color;
+  padding: 25px;
+
+  & .form-control {
+    background-color: #fff;
+  }
+}
+.modal-footer {
+  border: 0;
+  padding: 20px;
+  text-align: center;
+  color: @fl-body-muted-color;
+}
+.modal-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;
+  .transition(opacity 0.2s);
+
+  &.active {
+    opacity: 1;
+    pointer-events: auto;
+  }
+}
+.form-centered {
+  text-align: center;
+
+  & .form-control, & .btn {
+    width: 220px;
+    margin: 0 auto;
+    text-align: center;
+  }
+}
+
+
+@media @phone {
+  .modal.fade {
+    opacity: 1;
+  }
+  .modal-backdrop.in {
+    opacity: 0;
+  }
+  .modal-dialog {
+    position: fixed;
+    left: 0;
+    right: 0;
+    bottom: 0;
+    top: 0;
+  }
+  .modal-dialog {
+    margin: 0;
+
+    .modal.fade & {
+      .transition(top 0.3s);
+      top: 100%;
+      .translate(0, 0);
+    }
+    .modal.in & {
+      top: 0;
+    }
+
+    &:before {
+      content: " ";
+      .toolbar();
+    }
+  }
+  .modal-content {
+    border-radius: 0;
+    border: 0;
+    min-height: 100vh;
+    padding-top: @mobile-header-height;
+    .box-shadow(none);
+  }
+  .modal-header {
+    padding: 0;
+    border: 0;
+    min-height: 0;
+
+    & h3 {
+      line-height: 36px;
+    }
+  }
+}
+
+@media @tablet, @desktop, @desktop-hd {
+  .modal-dialog {
+    margin: 120px auto;
+
+    & .close {
+      position: absolute;
+      right: 5px;
+      top: 5px;
+    }
+  }
+  .modal-content {
+    border: 0;
+    border-radius: @border-radius-base;
+    .box-shadow(0 7px 15px @fl-shadow-color);
+  }
+  .modal-sm {
+    width: 375px;
+  }
+  .modal-header {
+    text-align: center;
+    border: 0;
+    padding: 25px;
+
+    & h3 {
+      font-size: 22px;
+      margin: 0;
+    }
+  }
+}