Permission migration

Related issue: https://labs.riseup.net/code/issues/4259

For now I just created the `cg:upgrade:user_permission` task that creates keys based on the old may_* profile flags.

It seemed to work when using a database created with the schema & fixtures from the 0.4.7.2 branch. The checkboxes on /me/permissions looked pretty much correct.

I'm currently moving some code from the rake task to the User model though, because it is hard to test it this way.