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/20170328163918_break_up_themes_table.rb

@@ -0,0 +1,54 @@
+class BreakUpThemesTable < ActiveRecord::Migration
+  def change
+    create_table :theme_fields do |t|
+      t.integer :theme_id, null: false
+      t.integer :target, null: false
+      t.string :name, null: false
+      t.text :value, null: false
+      t.text :value_baked
+      t.timestamps
+    end
+
+    add_index :theme_fields, [:theme_id, :target, :name], unique: true
+
+    [
+      [0, "embedded_scss", "embedded_scss"],
+      [0, "common_scss", "scss"],
+      [1, "desktop_scss", "scss"],
+      [2, "mobile_scss", "scss"],
+      [0, "head_tag", "head_tag"],
+      [0, "body_tag", "body_tag"],
+      [1, "header", "header"],
+      [2, "mobile_header", "header"],
+      [1, "top", "after_header"],
+      [2, "mobile_top", "after_header"],
+      [1, "footer", "footer"],
+      [2, "mobile_footer", "footer"],
+    ].each do |target, value, name|
+
+      execute <<SQL
+      INSERT INTO theme_fields (
+        theme_id,
+        target,
+        name,
+        value,
+        created_at,
+        updated_at
+      )
+      SELECT id, #{target}, '#{name}', #{value}, created_at, updated_at
+      FROM themes WHERE #{value} IS NOT NULL AND LENGTH(BTRIM(#{value})) > 0
+SQL
+      remove_column :themes, value
+    end
+
+    %w{ head_tag_baked
+        body_tag_baked
+        header_baked
+        footer_baked
+        mobile_footer_baked
+        mobile_header_baked
+       }.each do |col|
+      remove_column :themes, col
+    end
+  end
+end