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/settings.less

@@ -0,0 +1,67 @@
+.settings {
+  margin-top: 5px;
+
+  & > ul {
+    list-style: none;
+    margin: 0;
+    padding: 0;
+
+    & > li {
+      margin-bottom: 40px;
+    }
+  }
+  & fieldset > ul {
+    list-style: none;
+    margin: 0;
+    padding: 0;
+  }
+}
+.settings-account {
+  & li {
+    display: inline-block;
+    margin-right: 5px;
+  }
+}
+.notification-grid {
+  & table {
+    background: @fl-body-control-bg;
+    border-radius: @border-radius-base;
+  }
+  & table td, & table th {
+    border-bottom: 1px solid @fl-body-bg;
+    color: @fl-body-control-color;
+  }
+  & td, & th, & .yesno {
+    padding: 10px 15px;
+  }
+  & thead {
+    & th {
+      text-align: center;
+      padding: 15px 25px;
+    }
+    & .fa {
+      display: block;
+      font-size: 14px;
+      width: auto;
+      margin-bottom: 5px;
+    }
+  }
+  & .yesno-cell {
+    padding: 0;
+  }
+  & .yesno-control {
+    display: block;
+  }
+  & .yesno {
+    text-align: center;
+    cursor: pointer;
+  }
+  & .yesno-cell.highlighted .yesno, & .yesno:hover {
+    &:not(.disabled) {
+      background: darken(@fl-body-control-bg, 4%);
+    }
+  }
+  & .toggle-group {
+    cursor: pointer;
+  }
+}