Allow written time descriptions in moderation commands

[emmiegit]

Instead of only allow a single integer designating the number of minutes, a description should be allowed. (See dateparser) Ensure it supports negative timestamps, as phrases such as 10 minutes are interpreted as referring to time in the past.

The empty string, a single dash (-), or amount referring to 0 minutes will be interpreted as not imposing any time restriction.

This should be applied to all the moderation commands, such as !ban, !mute, etc.