Adds a alchemy:prepare rake task.

It creates the database, installs alchemy migrations, migrates and seeds alchemy data.
Using extend instead of include for sass mixins.
Mount Alchemy in dummy app to '/', not '/alchemy' anymore.
Adds a compressed migration for Alchemy 2.5 schema.
Fixes error message rendering on pages#update.
added i18n for Page in clipboard view form, fixed legacy url creation for external links