Add Ability to Push Cleanup Commits to PRs based on comments

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Problem

We would like to be able to run simple cleanup scripts against PRs without having to use the command line.

Proposed Solution

We should allow maintainers to make "magic comments", similar to how the bots work in conda-forge.
A repo could provide a set of comments that are applicable, and the script that should be run for the given comment.
The bot would look for a matching comment on an issue_comment event, and if the user has an author_association of OWNER, COLLABORATOR, or MEMBER, it will do the following:

• Post a comment to the issue signifying that the script is running
  • If the user is not authorized, post an issue comment and bail
• Check out the branch in the pull request in a temporary directory
• Run the appropriate script (or list of scripts)
• Commit the message using its own Git credentials
• Push the commit to the PR
• Delete the checkout

We could decide that some common phrases are always available, like @jupyterlab-bot, please run pre-commit could always trigger a pre-commit run --all-files with no configuration necessary in the repo.

Another option is to have a @jupyterlab-bot, please auto format, and have that by default run pre-commit run --all-files, but allow config to override.

Note: get a issue_comment event for testing, add a webhook to a repo, selecting "Issue comments". Add a comment to an issue, then grab the payload from the resulting event.

Additional context

We added a limited version of this in jupyterlab/maintainer-tools#39, but it would be cleaner to do it here.

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Alternative solution: scientific-python/MeeseeksDev#58

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Closing in favor of meeseeks