Commit model

We currently do not have Commit model where we can persist commits.

We do not have it, probably, because of a lot of commits that should be stored there, but it may be a good idea to consider adding this model, because of some benefits, like:

• consistent associations with other models
• removing additional code that handles this exception of not having this association (like non-existing noteable, as we can comment on commits)
• better handling of dangled commits (we currently lose a discussion on particular commit after it becomes dangled)
• reproducible changes history
• better association with Ci::Commit, now Pipeline
• reducing lookup on repository (repository.commit) for just the sake of checking if commit exists

/cc @DouweM