Add option for 24 hour clock (23:59 instead of 11:59 PM)

This certainly is a minor issue, but as someone who never had to deal with AM/PM for more than 5 minutes in their entire life, it confuses me more than it should. ;-) At least in the German-speaking world, writing `23:59` is much more common than `11:59 PM` (never seen that at all, in fact).

I guess the option would fit well under `Settings` - `Display` - `Appearance`.

Start by locating Settings → Display → Appearance in the Keybase client and trace the existing time-display behavior. Determine how a 12/24-hour preference would fit there. Done means users can choose a 24-hour clock and see times such as 23:59 instead of 11:59 PM.

Written by the indexing model from the issue text.