Comment when no assignee on MR

[julesdepoi]

Proposed feature: if MR assigneeId is null, grab a list of repo members and randomly suggest one of them to assign to.

Further design questions to discuss:

• could either be a potential enhancement for existing BotAction (TooManyAssigned or SelfMerge) or a standalone BotAction
• limitations of GitLab API in grabbing repo members by project ID only - members with inherited permissions from group will not show up
• how would the presence of a CODEOWNERS file affect this feature?