increment hotline message ID's per-channel
behavior
admins of a hotline-enabled channel currently see the following:
[HOTLINE #1345]
blah blah blah
when they create a channel, we want them to see low-numbered hotline message ID's:
[HOTLINE #1]
blah blah blah
this puts less administrative load/headache for admins of a busy channel when they need to respond.