Denormalise banned users on merge requests table

What does this MR do and why?

Denormalise banned users on merge requests table. Will drop about 250ms off of the merge requests list page on .com, e.g. https://gitlab.com/gitlab-org/gitlab/-/merge_requests.

This adds an author_banned boolean to the merge_requests table which is updated as users are banned/unbanned, which then vastly simplifies the query to fetch only merge requests by unbanned users. This also adds an index to cover the adjusted query.

Changelog: performance

Closes Denormalise banned users on merge requests table (#432774)

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