Feature Flag schema at CartoDB

This respects the documented usage, although loading feature flags from the local database.
Next step is Central sync.

@rambo, @Kartones , @rilla could you take a look? I'll do the sync logic meanwhile.
Please, move this to the ["Troubleshoting wiki page"](https://github.com/CartoDB/cartodb/wiki/Problems-faced-during-CartoDB-install-&-solutions-if-known) (it's a bit hidden but present around the end of the README)