DEV: extract inline js when baking theme fields (#6447)

* extract inline js when baking theme fields
* destroy javascript cache when destroying theme fields

This work is needed to support CSP work

db/migrate/20180927135248_create_javascript_caches.rb

@@ -0,0 +1,10 @@
+class CreateJavascriptCaches < ActiveRecord::Migration[5.2]
+  def change
+    create_table :javascript_caches do |t|
+      t.references :theme_field, null: false
+      t.string :digest, null: true, index: true
+      t.text :content, null: false
+      t.timestamps
+    end
+  end
+end