FEATURE: Native theme support

This feature introduces the concept of themes. Themes are an evolution
of site customizations.

Themes introduce two very big conceptual changes:

- A theme may include other "child themes", children can include grand
children and so on.

- A theme may specify a color scheme

The change does away with the idea of "enabled" color schemes.

It also adds a bunch of big niceties like

- You can source a theme from a git repo

- History for themes is much improved

- You can only have a single enabled theme. Themes can be selected by
    users, if you opt for it.

On a technical level this change comes with a whole bunch of goodies

- All CSS is now compiled using a custom pipeline that uses libsass
    see /lib/stylesheet

- There is a single pipeline for css compilation (in the past we used
    one for customizations and another one for the rest of the app

- The stylesheet pipeline is now divorced of sprockets, there is no
   reliance on sprockets for CSS bundling

- CSS is generated with source maps everywhere (including themes) this
    makes debugging much easier

- Our "live reloader" is smarter and avoid a flash of unstyled content
   we run a file watcher in "puma" in dev so you no longer need to run
   rake autospec to watch for CSS changes

app/assets/javascripts/admin/components/ace-editor.js.es6

@@ -14,6 +14,13 @@ export default Ember.Component.extend({
     }
   },
 
+  @observes('mode')
+  modeChanged() {
+    if (this._editor && !this._skipContentChangeEvent) {
+      this._editor.getSession().setMode("ace/mode/" + this.get('mode'));
+    }
+  },
+
   _destroyEditor: function() {
     if (this._editor) {
       this._editor.destroy();
@@ -41,6 +48,7 @@ export default Ember.Component.extend({
 
         editor.setTheme("ace/theme/chrome");
         editor.setShowPrintMargin(false);
+        editor.setOptions({fontSize: "14px"});
         editor.getSession().setMode("ace/mode/" + this.get('mode'));
         editor.on('change', () => {
           this._skipContentChangeEvent = true;