Improve ban command purge duration behavior

[hyper-neutrino]

The ban command currently just accepts a number of days. If an invalid number of days is provided (e.g. more than 7), a generic fatal error will be shown and the command will crash and be logged.

Now that Discord allows bans to purge any number of seconds between 0 and 7 days, we should improve this to parse the user's input to determine exactly how much time, and a proper error messages should be shown for invalid inputs.