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/adapters/theme.js.es6

@@ -0,0 +1,20 @@
+import RestAdapter from 'discourse/adapters/rest';
+
+export default RestAdapter.extend({
+  basePath() {
+    return "/admin/";
+  },
+
+  afterFindAll(results) {
+    let map = {};
+    results.forEach(theme => {map[theme.id] = theme;});
+    results.forEach(theme => {
+      let mapped = theme.get("child_themes") || [];
+      mapped = mapped.map(t => map[t.id]);
+      theme.set("childThemes", mapped);
+    });
+    return results;
+  },
+
+  jsonMode: true
+});