Attach commit

This MR adds the ability to attach GitLab commits to a card. Would love to use that feature in the trello-power-up.

I tried to change as little as possible on the existing code. Following is a short summary of what this MR consists:

• New card button to start attaching commit workflow
• Encapsulated commit functionality within Commit object (preventing polluting global namespace)
• Combined GitLab attachment section (contains attached MRs as well as commits)
• Small adaptions to existing code to help DRY

I didn't invest much time into refactoring since I don't know the state of #31 which probably will change some existing code.

Feedback appreciated & happy to help on other issues if needed.

Cheers

--jonas