[edxapp] Make migrations optional  #440

Feature Request

Is your feature request related to a problem or unsupported use case? Please describe.
Mysql does not support DDL transactions. As a result, running Django migrations in production is risky and should not be done without precautions.

Describe the solution you'd like
A first approach solution would be to allow deactivating the db_migrate jobs so that they can be activated voluntarily when a migration is required and initiated by a DBA.