FEATURE: Theme settings (2) (#5611)

Allows theme authors to specify custom theme settings for the theme. 

Centralizes the theme/site settings into a single construct

db/migrate/20180118215249_create_theme_settings.rb

@@ -0,0 +1,12 @@
+class CreateThemeSettings < ActiveRecord::Migration[5.1]
+  def change
+    create_table :theme_settings do |t|
+      t.string :name, limit: 255, null: false
+      t.integer :data_type, null: false
+      t.text :value
+      t.integer :theme_id, null: false
+
+      t.timestamps null: false
+    end
+  end
+end