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/models/stylesheet_cache.rb

@@ -3,11 +3,11 @@ class StylesheetCache < ActiveRecord::Base
 
   MAX_TO_KEEP = 50
 
-  def self.add(target,digest,content)
+  def self.add(target,digest,content,source_map)
 
     return false if where(target: target, digest: digest).exists?
 
-    success = create(target: target, digest: digest, content: content)
+    success = create(target: target, digest: digest, content: content, source_map: source_map)
 
     count = StylesheetCache.count
     if count > MAX_TO_KEEP
@@ -39,6 +39,8 @@ end
 #  content    :text             not null
 #  created_at :datetime
 #  updated_at :datetime
+#  theme_id   :integer          default(-1), not null
+#  source_map :text
 #
 # Indexes
 #