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

config/environments/development.rb

@@ -29,7 +29,6 @@ Discourse::Application.configure do
   config.active_record.migration_error = :page_load
   config.watchable_dirs['lib'] = [:rb]
 
-  config.sass.debug_info = false
   config.handlebars.precompile = false
 
   # we recommend you use mailcatcher https://github.com/sj26/mailcatcher
@@ -49,6 +48,13 @@ Discourse::Application.configure do
   config.enable_anon_caching = false
   require 'rbtrace'
 
+
+  require 'stylesheet/watcher'
+  if defined? Puma
+    STDERR.puts "Staring CSS change watcher"
+    @watcher = Stylesheet::Watcher.watch
+  end
+
   if emails = GlobalSetting.developer_emails
     config.developer_emails = emails.split(",").map(&:downcase).map(&:strip)
   end