Add ability to search for and find merge requests that are unassigned, or that don't have a review assigned

Proposal

Expand the user Merge Request (MR) search ability to be able to find:

1. MRs that are unassigned
2. MRs that do not have a reviewer

As a developing manager I often have many merge requests assigned to me, and I would like a quick and easy way to find merge requests that don't have a reviewer.

I know I can look at every single line in the output and click on the one that don't have a reviewer icon, but that is too slow and cumbersome. I want to have a query/filter that will return just the MR's that don't have a reviewer.

I see that there is the Reviewer search field but it only allows "=" or "!=" to an individual.

This needs something like Reviewer = unassigned or Reviewer != assigned or Reviewer = {empty,null}

Similarly, as a administrator, I often want to find all MRs that are currently unassigned. These MRs end up as plaque that need to be cleaned up. I want a filter that makes it easy to find these, similar to the Reviewer query above, where this time I'm looking for Assignee