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

lib/tasks/assets.rake

@@ -50,10 +50,8 @@ task 'assets:precompile:css' => 'environment' do
       # css will get precompiled during first request instead in that case.
 
       if ActiveRecord::Base.connection.table_exists?(ColorScheme.table_name)
-        STDERR.puts "Compiling css for #{db}"
-        [:desktop, :mobile, :desktop_rtl, :mobile_rtl].each do |target|
-          STDERR.puts "target: #{target} #{DiscourseStylesheets.compile(target)}"
-        end
+        STDERR.puts "Compiling css for #{db} #{Time.zone.now}"
+        Stylesheet::Manager.precompile_css
       end
     end