Define PR/MR label behavior

[mhutter]

One thing that caught my eye during #175

• The GitLab implementation will ensure the configured labels are added to the PR when the PR is created, but will NOT add any labels if there is already an open PR around.
• The GitHub implementation will always update the existing PR - and fail horribly from looking at the code (note this was already broken before the PR)

How should the behavior be?

1. Only ensure the label is set when creating the MR/PR and be done with it
2. Ensure the specified labels are set even if an MR/PR is already present

I can send a PR for the actual changes.