Hotfix: tweaks to old south migrations to allow them to be played on InnoDB tables, for Rackspace deployment

Two issues arose trying to run the migrations on the new Rackspace servers, due to some stricter DB constraints imposed by the InnoDB tables preferred by their cloud db server:

1. Language fields on log model had already been added in an earlier, parallel migration. Commented them out.
2. The `public_key` field on the `RegisteredDevicePublicKey` model was, early on, the primary key, but later the model was deleted and reinstated with `public_key` not as the primary key. The indexing of such a long field was pissing off InnoDB, so some revisionism allows us to pretend it was never a primary key.

There also seems to be a commit from @aronasorman in here, I guess because it was never cross-merged back to the release branches from master.