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/forms.less

@@ -0,0 +1,153 @@
+.form-group {
+  margin-bottom: 12px;
+}
+.form-control {
+  .box-shadow(none);
+  &:focus,
+  &.focus {
+    background-color: #fff;
+    color: @fl-body-color;
+    .box-shadow(none);
+  }
+}
+legend {
+  font-size: 14px;
+  border: 0;
+  font-weight: bold;
+  margin-bottom: 10px;
+}
+
+// Search inputs
+// @todo Extract some of this into header-specific definitions
+.search-input {
+  overflow: hidden;
+
+  &:before {
+    .fa();
+    content: @fa-var-search;
+    float: left;
+    margin-right: -36px;
+    width: 36px;
+    font-size: 14px;
+    text-align: center;
+    color: @fl-body-muted-color;
+    position: relative;
+    padding: @padding-base-vertical - 1 0;
+    line-height: @line-height-base;
+    pointer-events: none;
+  }
+  & .form-control {
+    float: left;
+    width: 225px;
+    padding-left: 36px;
+    padding-right: 36px;
+    .transition(~"all 0.4s");
+  }
+  & .clear {
+    float: left;
+    margin-left: -36px;
+    vertical-align: top;
+    opacity: 0;
+    width: 36px !important;
+    .rotate(-180deg);
+    .transition(~"transform 0.2s, opacity 0.2s");
+  }
+  &.clearable .clear {
+    opacity: 1;
+    .rotate(0deg);
+  }
+}
+
+// Select inputs
+.select-input {
+  display: inline-block;
+  vertical-align: middle;
+}
+.select-input select {
+  display: inline-block;
+  width: auto;
+  -webkit-appearance: none;
+  padding-right: @padding-base-horizontal + 16;
+  cursor: pointer;
+}
+.select-input .fa {
+  margin-left: -@padding-base-horizontal - 16;
+  pointer-events: none;
+  color: @fl-body-muted-color;
+}
+
+.checkbox-switch {
+  & label {
+    padding-left: 65px;
+  }
+  & .switch-control {
+    float: left;
+    margin-left: -65px;
+    margin-top: -4px;
+  }
+  & .loading-indicator {
+    display: inline-block;
+    margin-left: 10px;
+  }
+}
+.switch-control, .yesno-control {
+  & input[type=checkbox] {
+    display: none;
+  }
+}
+.switch {
+  width: 50px;
+  height: 28px;
+  padding: 3px;
+  position: relative;
+  border-radius: 14px;
+  background: @fl-body-control-bg;
+  .transition(background-color 0.2s);
+
+  input:checked + & {
+    background: @fl-body-primary-color;
+  }
+
+  & .loading-indicator {
+    opacity: 0;
+
+    .loading& {
+      opacity: 1;
+    }
+  }
+  &:before, & .loading-indicator {
+    position: absolute;
+    width: 22px;
+    height: 22px;
+    padding: 0;
+    left: 3px;
+    .transition(~"opacity 0.2s, left 0.2s");
+
+    input:checked + & {
+      left: 25px;
+    }
+  }
+  &:before {
+    content: ' ';
+    background: @fl-body-bg;
+    border-radius: 11px;
+    box-shadow: 0 2px 4px @fl-shadow-color;
+  }
+}
+.yesno-control {
+  cursor: pointer;
+  margin: 0;
+}
+.yesno {
+  font-size: 14px;
+
+  &.yes {
+    color: #58A400;
+  }
+  &.no {
+    color: #D0021B;
+  }
+  &.disabled {
+    color: @fl-body-muted-more-color !important;
+  }
+}