Rename tasks

Created by: riyad

Rename rake tasks to be more consistent:

• rename gitlab:app:backup_* to gitlab:backup:*
• rename gitlab:app:enable_automerge to gitlab:enable_automerge
• rename gitlab:activate_namespaces to gitlab:enable_namespaces
• put add_user*_to_project_teams bulk user import tasks under gitlab:import:...
• add gitlab:satellites:create for clarity