Track updates on branch_names

[thepetk]

Right now the MR titles and branch_names are configured from the user in the g2s.yaml. If someone re-configures them after the deployment of gitlab2sentry the module won't be able to track the old names.

Expected Behavior: gitlab2sentry would be able to track every update and always check for branch_names or MRs with previous names

Possible Solution: Refactore the title and branch_name attributes to be lists. The first item of the list will be the active name and all the previous ones will be checked as previous names.