Make reason mandatory to reject a new user?

[yarikoptic]

#1301 added rejection reason. I often click on Reject for some reason assuming it would somehow give me a dialog to provide a reason... sure thing it doesn't and thus I reject without stating a reason. I think it is reasonable to expect to provide reason for every rejection, thus should we make Reject disabled unless reason is specified or add some check to not actually do the action and give error message if no reason is given?

[waxlamp]

@mvandenburgh can you work on this? I think it should be pretty simple--if it's not, please report back here.

[dandibot]

🚀 Issue was released in v0.3.12 🚀