Drop down for options on filters

When filtering using the UI in the SQB, it is hard to know what options there are and can result in a user giving up. It would be useful to have this as a drop-down on the fields that have a reasonably limited number of options.

Start by locating the SQB filtering UI and the fields that currently accept free-form input. Identify which fields have a reasonably limited set of options and check how their values are supplied. Done means those fields offer discoverable dropdown choices without making existing filtering behavior harder to use.

Written by the indexing model from the issue text.