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

Gemfile.lock

@@ -105,7 +105,7 @@ GEM
       rake
       rake-compiler
     fast_xs (0.8.0)
-    ffi (1.9.17)
+    ffi (1.9.18)
     flamegraph (0.9.5)
     foreman (0.82.0)
       thor (~> 0.19.1)
@@ -318,13 +318,11 @@ GEM
       crass (~> 1.0.2)
       nokogiri (>= 1.4.4)
       nokogumbo (~> 1.4.1)
-    sass (3.2.19)
-    sass-rails (5.0.4)
-      railties (>= 4.0.0, < 5.0)
-      sass (~> 3.1)
-      sprockets (>= 2.8, < 4.0)
-      sprockets-rails (>= 2.0, < 4.0)
-      tilt (>= 1.1, < 3)
+    sass (3.4.23)
+    sassc (1.11.2)
+      bundler
+      ffi (~> 1.9.6)
+      sass (>= 3.3.0)
     seed-fu (2.3.5)
       activerecord (>= 3.1, < 4.3)
       activesupport (>= 3.1, < 4.3)
@@ -462,8 +460,7 @@ DEPENDENCIES
   rtlit
   ruby-readability
   sanitize
-  sass
-  sass-rails
+  sassc
   seed-fu (~> 2.3.5)
   shoulda
   sidekiq