handle r?

• if not in reviewer_group, refuses
• set commit "review" status to running
• if configured, add need_review_label label to MR (except MR author)
• if configured, assign random user from reviewer_group to MR
• add a comment to the MR ? (eg: thx @author, @reviewer will take a look at your contribution)

ps: needs to add need_review_label & reviewer_group (nullable) column