Bugzilla’s current flag UI is a `<select>` dropdown list, but given that a flag is a simple quad state field with short label values (unset, `?`, `+`, or `-`), it’s better to use buttons instead for easier selection. Usage of dropdown lists should be avoided whenever possible especially when the values are not many. Implementation-wise, these buttons will be elements having the `radio` ARIA role, not `button`. See the attached image for a quick mockup.

FYI review flags for Firefox and related products will be going away as all reviews are moved to Phabricator. We'll be wanting to move the remaining flags (e.g. uplift-approval flags) at some point as well (not entirely sure how, maybe as revision tags). So although this will be useful for any other products still using attachment flags, the overall impact will be lower than it might have been in the past.

I know the review flag is going away, but give that there are still other various quad state fields, this is worth doing. Part of the style has already been added in Bug 1489718.

Screencast: https://twitter.com/BugzillaUX/status/1046941525928685568