Bot prefix causes errors if set to `<`

[Exper1mental]

If the bot prefix is set to <, it interprets users being mentioned (when at the beginning of the message) as commands to be executed, which yields the obvious Command Not Found error.

[Jay-Madden]

As discussed in discord this isnt actually a bug with clembot but more just a result of how discord internally represents mentions, we should just disallow this prefix entirely to prevent it

[jkriste]

Completed via #871