Let halfops change the channel topic

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

Improve auditability of sensitive operator actions

strip local_ from oper capab names, also consolidate unban into ban

checkAlwaysOnExpirationNoMutex was respecting registered status, but
always-on clients were not considered registered at the time of the
initial check, so they were being created regardless of expiration.

Add the new utf8-only cap, disallow non-utf8 when websockets are enabled

enable the use of grouped nicks as account names for SASL,
if force-nick-equals-account is disabled

Give Tor clients who authenticate via SASL a unique cloak, so chanops
can ban *!*@tor-network.onion and still allow authenticated Tor users

HELP responses weren't taking the client nickname as a parameter,
as is standard.

Fixes #810

1. Consolidate PURGE and UNPURGE into subcommands
2. Add PURGE LIST
3. PURGE ADD now requires a confirmation code

Fixes #1294

Channel rename (both of registered and unregistered channels) would leave
the old name unreclaimable.

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.

This didn't work correctly for IPv6 or custom nets.
/UNDLINE IP can temporarily be used to reset the throttle.

Validate imported databases for confusable nicks

Tor listeners should never see an STS cap.

Add an undocumented 'hide-sts' key for listeners that hides the STS cap.
This can be used if the listener is secured at layer 3 or 4 (VPNs,
E2E mixnets). It will be necessary to add the relevant IPs to `secure-nets`.

Allow custom limit definitions that encompass multiple subnets