RELAYMSG should respect mutes

Implements #1577, but the issue should remain open until we clean up
the debugging loglines.

Transition most "is an operator" checks to require a specific operator
capability

Let halfops change the channel topic

Registered channels should be eagerly created on startup, and should
remain (and be visible in LIST) even when they have no members.

Track channel join times, use them to optionally enforce history access
restrictions

Fixes #1260

INVITE playback (#1409) was buggy, due to the double use of (Item).Params[0]
for the channel name and the recipient nick. Stuff the channel name in
(Item).Message.Message instead.

Store the channel-user modes of always-on clients along with their
channel memberships, restore them on server startup. This will coexist
alongside /CS AMODE, which autoapplies modes to clients on join regardless
of their always-on status.

Record INVITE in DM history for the benefit of offline always-on clients

Instead of building a new serialized message for each recipient,
try to cache them.

Resuming clients without the resume capability would receive
two JOIN lines per channel.

See #1171.

Add chanmode preventing speech to error message.

Fixes #1182.

1. TAGMSG were incorrectly being considered multilines, because
   Is512() was checking the wrong thing
2. Playback of TAGMSG should depend on event-playback, not on message-tags

This affects /TOPIC, /INVITE, and a few others.

This propagates CS AMODE changes to the actual modes, but not the other
way around.

Also fixes #909.