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/discourse-common/components/combo-box.js.es6

@@ -22,17 +22,40 @@ export default Ember.Component.extend(bufferedRender({
     let selected = this.get('value');
     if (!Em.isNone(selected)) { selected = selected.toString(); }
 
-    if (this.get('content')) {
-      this.get('content').forEach(o => {
+    let selectedFound = false;
+    let firstVal = undefined;
+    const content = this.get('content');
+
+    if (content) {
+      let first = true;
+      content.forEach(o => {
         let val = o[this.get('valueAttribute')];
         if (typeof val === "undefined") { val = o; }
         if (!Em.isNone(val)) { val = val.toString(); }
 
         const selectedText = (val === selected) ? "selected" : "";
         const name = Handlebars.Utils.escapeExpression(Ember.get(o, nameProperty) || o);
+
+        if (val === selected) {
+          selectedFound = true;
+        }
+        if (first) {
+          firstVal = val;
+          first = false;
+        }
         buffer.push(`<option ${selectedText} value="${val}">${name}</option>`);
       });
     }
+
+    if (!selectedFound) {
+      if (none) {
+        this.set('value', null);
+      } else {
+        this.set('value', firstVal);
+      }
+    }
+
+    Ember.run.scheduleOnce('afterRender', this, this._updateSelect2);
   },
 
   @observes('value')
@@ -81,9 +104,14 @@ export default Ember.Component.extend(bufferedRender({
       }
       this.set('value', val);
     });
+
     Ember.run.scheduleOnce('afterRender', this, this._triggerChange);
   },
 
+  _updateSelect2() {
+    this.$().trigger('change.select2');
+  },
+
   _triggerChange() {
     this.$().trigger('change');
   },