Request review from non-collaborators by mentioning them

[Qrox]

Fixes #98. Github restricts requested reviewers to people having write access to a repo (collaborators), but people may actually want to request non-collaborators for review.

This change separates non-collaborators from collaborators and mentions the non-collaborators with a review comment instead. To avoid repeatedly mentioning the same people, it now checks the review comments for people who already left a review or were pinged by the action and skip them along with the already requested reviewers when requesting new reviews. It also skips the PR author in case the author is not a collaborator.

A new test case is added to test the newly added functions. I also tested them in Qrox/Cataclysm-DDA#14 and Qrox/Cataclysm-DDA#15 (more detail in CleverRaven/Cataclysm-DDA#66072).