Invert merge_requests_author_approval to match similar fields

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#324087 (comment 585540027)

merge_requests_author_approval is at odds with its natural partner merge_requests_disable_committers_approval as well as with it's application_settings cousins prevent_merge_requests_author_approval and prevent_merge_requests_committers_approval in that true means "allow this action". This caused QUITE a bit of confusion in #324087 (closed). To fix this, we need to go through several steps:

• add a new column merge_requests_disable_author_approval
• add a method to populate/change it when merge_requests_author_approval is changed
• check for any existing indices that need to be reconstituted with the new field
• a background migration to backfill the new field for every single project
• alias Project#merge_requests_author_approval (or replace every call with the new attribute)
• invert any logic
• drop merge_requests_author_approval and any associated indices

It's a lot of busy work, but not pressing at this point, I don't think.. actually, a really good Community contribution candidate (or for the hackathon)