Merge commands to add settings into one, introduce custom prefixes

[nonchris]

main change

There is now one command for all settings called f!set.
It unifies:

• The old set command for log and archive
• The set-voice-channel command
• The allow-edit command

The switching between those different settings is done in the main function of the set-command.
It uses the help of a few extra functions called from there.

All commands work the same way:
[prefix]command [name of setting] [settings value]

other changes

• Bot now supports custom prefixes (can be set via set command)
• Bot now listens for mentions as prefix
• Improve on_command_error() listener