CI: Adding GitHub action to assign issues based on comment
Currently, only maintainers and triagers can assign issues.

This implements a hack, so when any contributor writes the keyword `take` in an issue, GitHub actions will automatically assign the issue to the commenter.

I think this should be very helpful once people start to assign issues to themselves, since adding the `Assigned to nobody` filter should let people find issues that nobody is working on. May be in the future we want to implement another action to unassign issues after a period of inactivity, but so far this should be much better than just having people adding comments.

@pandas-dev/pandas-core what do you think?
