Add ability to search for users by roles in the admin

[GUI]

We had a request today for the ability to search for api users in the admin by the roles they have assigned. I think this request may have come up before, but in any case, it seems like a good idea, since you may want to find all the users you've granted special access to (for example, all the API keys that have protected write access).

[GUI]

Added in NREL/api-umbrella@edead15

This makes the role information part of the free-form search that is already in the admin interface. So it's not a specific filter for exact roles, but I think this should hopefully do the trick as a first pass.