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/code.less

@@ -0,0 +1,69 @@
+//
+// Code (inline and block)
+// --------------------------------------------------
+
+
+// Inline and block code styles
+code,
+kbd,
+pre,
+samp {
+  font-family: @font-family-monospace;
+}
+
+// Inline code
+code {
+  padding: 2px 4px;
+  font-size: 90%;
+  color: @code-color;
+  background-color: @code-bg;
+  border-radius: @border-radius-base;
+}
+
+// User input typically entered via keyboard
+kbd {
+  padding: 2px 4px;
+  font-size: 90%;
+  color: @kbd-color;
+  background-color: @kbd-bg;
+  border-radius: @border-radius-small;
+  box-shadow: inset 0 -1px 0 rgba(0,0,0,.25);
+
+  kbd {
+    padding: 0;
+    font-size: 100%;
+    font-weight: bold;
+    box-shadow: none;
+  }
+}
+
+// Blocks of code
+pre {
+  display: block;
+  padding: ((@line-height-computed - 1) / 2);
+  margin: 0 0 (@line-height-computed / 2);
+  font-size: (@font-size-base - 1); // 14px to 13px
+  line-height: @line-height-base;
+  word-break: break-all;
+  word-wrap: break-word;
+  color: @pre-color;
+  background-color: @pre-bg;
+  border: 1px solid @pre-border-color;
+  border-radius: @border-radius-base;
+
+  // Account for some code outputs that place code tags in pre tags
+  code {
+    padding: 0;
+    font-size: inherit;
+    color: inherit;
+    white-space: pre-wrap;
+    background-color: transparent;
+    border-radius: 0;
+  }
+}
+
+// Enable scrollable blocks of code
+.pre-scrollable {
+  max-height: @pre-scrollable-max-height;
+  overflow-y: scroll;
+}