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/controllers/application_controller.rb

@@ -411,8 +411,8 @@ class ApplicationController < ActionController::Base
     def custom_html_json
       target = view_context.mobile_view? ? :mobile : :desktop
       data = {
-        top: SiteCustomization.custom_top(session[:preview_style], target),
-        footer: SiteCustomization.custom_footer(session[:preview_style], target)
+        top: Theme.lookup_field(session[:preview_style], target, "after_header"),
+        footer: Theme.lookup_field(session[:preview_style], target, "footer")
       }
 
       if DiscoursePluginRegistry.custom_html