atlas_git_merge: new release branch merge script

Add a new (Python) git merge script for release branch merging. The script tries to automatically handle the sweep:ignore MRs. The corresponding warning from the release notes script has been removed.

Usage (same as before):

./Build/AtlasBuildScripts/atlas_git_merge.py 24.0

This tries to implement the prescription from @tsulaia that was discussed over e-mail. cc @atlas-release-coordinators

Note: For testing this new script it's easiest to have a separate athena checkout of this branch.