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/assets/stylesheets/common/foundation/base.scss

@@ -1,5 +1,5 @@
-@import "common/foundation/variables";
-@import "common/foundation/mixins";
+@import "./variables";
+@import "./mixins";
 
 // --------------------------------------------------
 // Base styles for HTML elements

app/assets/stylesheets/common/foundation/variables.scss

@@ -28,7 +28,7 @@ $base-font-size: 14px !default;
 $base-line-height: 19px !default;
 $base-font-family: Helvetica, Arial, sans-serif !default;
 
-/* These files don't actually exist. They're injected by DiscourseSassImporter. */
+/* These files don't actually exist. They're injected by Stylesheet::Compiler. */
 @import "theme_variables";
 @import "plugins_variables";
 @import "common/foundation/math";