workflows: Make issue-subscriber more robust for labels with special characters

Also, replace the existing actionscript implementation with a python
script that can be run outside of GitHub Actions.

workflows: Make issue-subscriber more robust for labels with special characters

Also, replace the existing actionscript implementation with a python
script that can be run outside of GitHub Actions. The intention is
that going forward, all github action functionality would be implemented
in this script.

Reviewed By: kwk

Differential Revision: https://reviews.llvm.org/D116762

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