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/bootstrap/mixins/grid-framework.less

@@ -0,0 +1,91 @@
+// Framework grid generation
+//
+// Used only by Bootstrap to generate the correct number of grid classes given
+// any value of `@grid-columns`.
+
+.make-grid-columns() {
+  // Common styles for all sizes of grid columns, widths 1-12
+  .col(@index) { // initial
+    @item: ~".col-xs-@{index}, .col-sm-@{index}, .col-md-@{index}, .col-lg-@{index}";
+    .col((@index + 1), @item);
+  }
+  .col(@index, @list) when (@index =< @grid-columns) { // general; "=<" isn't a typo
+    @item: ~".col-xs-@{index}, .col-sm-@{index}, .col-md-@{index}, .col-lg-@{index}";
+    .col((@index + 1), ~"@{list}, @{item}");
+  }
+  .col(@index, @list) when (@index > @grid-columns) { // terminal
+    @{list} {
+      position: relative;
+      // Prevent columns from collapsing when empty
+      min-height: 1px;
+      // Inner gutter via padding
+      padding-left:  (@grid-gutter-width / 2);
+      padding-right: (@grid-gutter-width / 2);
+    }
+  }
+  .col(1); // kickstart it
+}
+
+.float-grid-columns(@class) {
+  .col(@index) { // initial
+    @item: ~".col-@{class}-@{index}";
+    .col((@index + 1), @item);
+  }
+  .col(@index, @list) when (@index =< @grid-columns) { // general
+    @item: ~".col-@{class}-@{index}";
+    .col((@index + 1), ~"@{list}, @{item}");
+  }
+  .col(@index, @list) when (@index > @grid-columns) { // terminal
+    @{list} {
+      float: left;
+    }
+  }
+  .col(1); // kickstart it
+}
+
+.calc-grid-column(@index, @class, @type) when (@type = width) and (@index > 0) {
+  .col-@{class}-@{index} {
+    width: percentage((@index / @grid-columns));
+  }
+}
+.calc-grid-column(@index, @class, @type) when (@type = push) and (@index > 0) {
+  .col-@{class}-push-@{index} {
+    left: percentage((@index / @grid-columns));
+  }
+}
+.calc-grid-column(@index, @class, @type) when (@type = push) and (@index = 0) {
+  .col-@{class}-push-0 {
+    left: auto;
+  }
+}
+.calc-grid-column(@index, @class, @type) when (@type = pull) and (@index > 0) {
+  .col-@{class}-pull-@{index} {
+    right: percentage((@index / @grid-columns));
+  }
+}
+.calc-grid-column(@index, @class, @type) when (@type = pull) and (@index = 0) {
+  .col-@{class}-pull-0 {
+    right: auto;
+  }
+}
+.calc-grid-column(@index, @class, @type) when (@type = offset) {
+  .col-@{class}-offset-@{index} {
+    margin-left: percentage((@index / @grid-columns));
+  }
+}
+
+// Basic looping in LESS
+.loop-grid-columns(@index, @class, @type) when (@index >= 0) {
+  .calc-grid-column(@index, @class, @type);
+  // next iteration
+  .loop-grid-columns((@index - 1), @class, @type);
+}
+
+// Create grid for specific class
+.make-grid(@class) {
+  .float-grid-columns(@class);
+  .loop-grid-columns(@grid-columns, @class, width);
+  .loop-grid-columns(@grid-columns, @class, pull);
+  .loop-grid-columns(@grid-columns, @class, push);
+  .loop-grid-columns(@grid-columns, @class, offset);
+}