channel founders should be able to disable channel history

[slingamn]

This is relevant for public servers, but also for internal team/corporate servers that want to designate unlogged channels.

[DanielOaks]

Definitely agree. Might even be worth making it explicit that chan history is being logged on join with a numeric or a message, if one isn't already defined by another server out there.

[slingamn]

dan says: "I think more of a 'let the channel ops set the max amount of history to store'"

Idea: /chanserv set #chan history-expiration [duration]. A zero duration disables history, a nonzero duration sets history to expire after that duration (and disables persistent history), no parameter resets to server defaults.

[DanielOaks]

supplemental note: /cs set is good, using that over entirely new /cs subcommands is nice <3

[slingamn]

Closed in #792