Prevent LUSERS stats from getting out of sync when modes are
modified on offline clients.

Allow snomasks to be added via oper config block, even if the oper
doesn't have `ban` or `snomasks` and therefore can't add snomasks
on their own.

ERR_INVALIDMODEPARAM should take both the mode and the invalid parameter
as arguments.

Make snomask add/remove behavior match other ircds

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

Fixes #1260

ParseUserModeChanges and ParseChannelModeChanges already validate
that the modes are modifiable, so there's no need to repeat it here.

Add chanmode preventing speech to error message.

Fixes #1182.

Previously, we were applying defaults before the user had completed
registration. This meant that the number of invisible users was
incremented when the user connected, and then the total was incremented
when registration was completed.

Now both counters are updated at the same time. If a user disconnects
prior to registration, +i has not yet been applied so it would not be
decremented.

This ensures that counts like the number of invisible users are updated
correctly.

Add default-user-modes configuration to conventional.yaml.
Fix comment on DefaultUserModes.

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

Also fixes #909.

Also add +C no-ctcp mode, since it's not conflicted

* Fix #684
* Fix #683
* Add `CHANSERV CLEAR`
* Allow mode changes from channel founders even when they aren't joined
* Operators with the chanreg capability are exempt from max-channels-per-account
* Small fixes and cleanup

Also ensure that people can remove their own persistent modes via CS AMODE

Also fix some issues with STATUSMSG

This moves channel registration to an eventual consistency model,
where the in-memory datastructures (Channel and ChannelManager)
are the exclusive source of truth, and updates to them get persisted
asynchronously to the DB.