m_banredirect Remove unnecessary hooks, cleanup

- Remove prioritization before m_banexception, it doesn't even hook OnUserPreJoin
- Remove OnChannelDelete handler: nothing to do in that case, our extension item is automatically freed
- Remove empty OnRehash handler
m_banredirect Deny the join to the redirect channel if it has a redirecting ban set that matches the user

Fixes #395 reported by @patatje368
m_messageflood Ask to be the last to handle OnUserPreMessage/OnUserPreNotice

Fixes #394 reported by @JDowny