Make MR creations via UI (already an "All events" activity) update "last_activity_at" API output

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Release notes

Problem to solve

Essentially the same as in Make Git branch and tag creations via UI (alrea... (#505462 - closed)+ but when opening a merge request:

In the UI this is considered an activity, but the API field last_activity_at is not updated by it.

Proposal

Update that API output field accordingly.

Intended users / Feature Usage Metrics / Does this feature require an audit event?

Same as in gitlab#505462