Task to create admins and fix in migration

* Alter migration so that users, with the (old) role of Admin and Organizer, become admins.
* Create task to make an admin based on user's email address

   Usage: rake user:admin["my@email.osem"]

I have used the update_coumns on purpose, so that 
  1. we skip validations and callbals and we don't run into validation errors for the User model, which is a bit outside of the scope of this operation. 
  2. In addition we do not alter the updated_at column, maintaining more accurate information of when the user altered his/her account.