Make the migration RemoveMetadataFromRooms faster
No need to generate room options in the migration, it's done on the fly

This migration was taking too long to run and generating problem with
inconsistent BigbluebuttonRoom's. Since the room options are generated when
first accessed if they don't exist, we don't really need to create them in
the migration.

The inconsistencies in BigbluebuttonRoom's will be solved later.

refs #987
Refactor failing migration after rails 4

Was failing because it tried to use Space#save, that tried to also save its
associated BigbluebuttonRoom, that failed for having the attribute #record
that's invalid in rails 4.
Had to use fake models instead. Couldn't use raw SQL because we also need to
generate the images according to CarrierWave, and it would not be ideal to
do it all manually.

refs #987
Fix the migration of attachments to a private folder

Was moving not only the attachments but all the logos from their
folder to the private folder. The logos should be kept there.

refs #987
Fix migration of events to set updated_at and created_at

Old events were replaced by events from MwebEvents, so they were all
created again. They were however all having the created_at and
updated_at values set to the time the migration ran, instead of
the time when the former event was created/updated.

refs #987