Regression fix: history entry should not be marked as hidden ...

... when change comes with a comment.
Fix regular expresion on migrate_attachments script.
Add routing_key required parameter to base and postgresql events backend.
Improve events api for fulfill the new events protocol of taiga-events.
Send userstories change event on bulk order is changed.
Add rabbitmq events backend.
Make events app play well with django 1.7 new app loading system.
Disable events signals on sample data.
WIP