Add github action to apply patches

[lucasdemarchi]

Add github-action that parses a comment like "/merge", filtered by users with commit rights, that does the following:

• Add Reviewed-by to the patches when a reviewed approves the PR. It's assumed that PR approval means all patches are reviewed
• Add a "Link: " as trailer
• Add s-o-b by whom commented /merge.

[lucasdemarchi]

Maybe relevant helpers I typed today: lucasdemarchi/toolbox@160a33a

Tested in #78, applying the patches:
git applypr -s -l 78 "Reviewed-by: Emil Velikov <[email protected]>"

I also force pushed the branch so the PR gets closed automatically when it's merged to master branch and the state transitions to "merged" rather than "closed".