Allow for configuration of intro message

[MineRobber9000]

For example, configuring the intro a discussion board for tilde.town to a discussion board for {yourserver.com}.

This would obsolete the tilde.team fork of BBJ (as it could just be configured to say tilde.team and not tilde.town.

[benharri]

mostly handled by 5c708b4

feel free to reopen if we need further configuration for this.

[MineRobber9000]

no, the idea is that the server should be able to control what the intro message says for "BBJ is a discussion board for {whatever}".