Combine `author` and `assignee` in homepage queries

Problem to Solve

Assignee may not give a full picture of the MRs a user is working on. It's possible that the user is only the author of the merge request, but still participating in some way.

Proposal

We should consider that for any queries where we're filtering for the assignee, we also check if that user is the author.

Additional Details

It's possible that this isn't true for all users and may create additional noise. We should do some investigation into this first to see if we can determine the disparity here.