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

db/migrate/20170324144456_amend_css_columns_in_theme.rb

@@ -0,0 +1,13 @@
+class AmendCssColumnsInTheme < ActiveRecord::Migration
+  def change
+    rename_column :themes, :stylesheet, :desktop_scss
+    rename_column :themes, :mobile_stylesheet, :mobile_scss
+    rename_column :themes, :embedded_css, :embedded_scss
+
+    add_column :themes, :common_scss, :text
+
+    remove_column :themes, :stylesheet_baked
+    remove_column :themes, :mobile_stylesheet_baked
+    remove_column :themes, :embedded_css_baked
+  end
+end