Consider changing search drop-downs to check box selects

At https://openoversight.lucyparsonslabs.com/find in particular, doesn't it make sense for Officer Rank and maybe Race to be checkboxes, so that more than one can be selected? Being able to effectively say, "I don't know what his rank was", or, "he was Asian or Pacific Islander" is nicer than having to do multiple separate queries.

And I assume lots of people will be like me and not have any idea what the officer's rank is, so maybe still defaulting to Commander makes sense, but also having a "Check all" or "I don't know" button that checks all would be good, or defaulting to all.

Start at the /find page and trace how the Officer Rank and Race dropdowns submit search criteria. Define the multi-selection and “I don't know” behavior described in the issue, then verify that searches using multiple or all values return the expected results.