Find merge requests that have previously been assigned to me

Why

I'd like to check up on MRs that I've reviewed.

Currently I can do this by searching my email for "Assigned to @jamedjo"

What

• Store history of merge request assignees
• Migrate historical merge request assignees. Possibly using data from Todo.where(action: Todo::ASSIGNED) and maybe merge_request.author_id if we want to include initial assignee
• Provide API to look up this information
• Allow merge request index to filter by a param such as prevously_assigned
• Allow filter by previous assignee from the UI