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

@@ -93,8 +93,6 @@ gem 'thor', require: false
 gem 'rest-client'
 gem 'rinku'
 gem 'sanitize'
-gem 'sass'
-gem 'sass-rails'
 gem 'sidekiq'
 gem 'sidekiq-statistic'
 
@@ -181,3 +179,5 @@ gem 'memory_profiler', require: false, platform: :mri
 gem 'rmmseg-cpp', require: false
 
 gem 'logster'
+
+gem 'sassc', require: false