Improve time handling in :next command about testdaybot

[FlorinMezei]

Some simple examples:

a) start time in the past
A simple check if start time is before current time - show an error saying something like: "Start time is set in the past - note that the timezone is UTC"

a) start time after end time
A simple check if start time is before end time - show an error saying something like: "Start time was set after End Time"

[xabolcs]

Can I take this?

[whimboo]

Sure! ;) I assigned you now.

[whimboo]

PR #77 has been merged. I will get all the latest changes merged to the release branch now, so they will be active on the running testdaybot instance in #qa in a bit.