Countdown-style discussion time

[tatablack]

Currently, the discussion time is shown as an increasing timer - a user asked for the option to make it a countdown instead.

After taking an initial look, this seems to require more changes to the code than I'd like, but it should still be doable.

[ghost]

Seconded. I notice that, unless one already knows Leaner Coffee uses 5 and only 5 minute blocks, a user has no idea how much time is left either when starting a discussion or as feedback as a discussion is going.