Migration: reintroduce account migration support

Reintroduce support for account migration removed in #154.

Revert commits:
* d5b2ad878fed5420bba8c326c69c94b874c2ef78
  * login method in AccountController.groovy and LoginController.groovy
  * re-add MigrateController
  * re-add virtualhome/grails-app/views/migrate
* 6a11b8378e820442350b3d93126b78a4e0b9f26c
  * add mappings for /migration/ to "migrate" controller
* 2d4d4649b1a543505d505eaf4b90984d12fa728f
  * AccountController, LoginController: re-add import of MigrateController