Broadcast configuration - servers should be able to opt in or out

[Nos78]

This issue relates to the per-server settings project, and so this work is dependent upon that project. Although this issue is small, it is quite important and so Im raising it as an issue.

Currently, the !broadcast command will broadcast to every server that MrData has access to, if he can find a channel with the appropriate permissions. To avoid abuse, only the bot owner can use this command, but that does not eliminate all the issues.

This may upset some users, who may not want a bot posting unsolicited messages onto their discord server! Particularly when this is used to advertise new features or services, and at first glance may appear to be an unsolicited advertisement.