Correct `nullable` values in models and migrations

* Region.description is declared as non-nullable in migration, so model nullable
value changed to False too.

* FederationProtocol.mapping_id must logically be not nullable. Provide a
migration script to add the NOT NULL constraint to the column.

Change-Id: I15a9e50b5461a112faca08f0ea68cbae2ce77d0d
Partial-Bug: #1292591
Merge "Correct `nullable` values in models and migrations"