Support for Pull Requests from forks

[henriquegemignani]

If using on: pull_request, when a pull requests comes from a fork the workflow doesn't get permission to make comments on the pull request.

I changed my workflow to use on: pull_request_target, however the messages no longer work properly. I believe it's because the head/base changes?

[mw-root]

I'm really not sure but that sounds like a reasonable explanation.

I'll have to do some research.