Add chanmode preventing speech to error message.

Fixes #1182.

CLIENTS LIST shows information about clients attached to a nick.
CLIENTS LOGOUT allows individual (or all) sessions to be logged out.

SESSIONS is now an alias for CLIENTS LIST.

Fixes #1072.

See #1240.

anything other than a (net.Error) with Temporary() == true
is treated as a fatal error that causes the http server to exit

See discussion on #68.

fix #1248

These should have been removed in #1027.

Link to the new draft PR:
<https://github.com/ircv3/ircv3-specifications/pull/420>

Changes in the spec:

- Use standard replies instead of numerics:
  <https://github.com/ircv3/ircv3-specifications/pull/420/files#diff-70e90beef48dc9cf5d784d1e179ea822R44>
- Allow RENAME to a different case:
  <https://github.com/ircv3/ircv3-specifications/pull/420/files#diff-70e90beef48dc9cf5d784d1e179ea822R42>

This commit makes oragono send the PART-JOIN fallback even on case-only
changes. This is so that clients don't have to worry about oragono's
UTF8 casefolding. See the following comments for further info:
<https://github.com/ircv3/ircv3-specifications/pull/420#issuecomment-668770837>

Misc fixes:

- Remove unused variable,
- Add missing calls to utils.SafeErrorParam,
- Don't fill replies with the user-provided "oldName", for the same
  reason as sending the PART-JOIN fallback.

Websocket listeners would process an arbitrary number of invalid
(non-text or blank) messages without throttling. This imposes fakelag
on such messages by treating them as blank lines.

Since allocating them is rare.

If force-nick-equals-account is disabled, then this could cause
a client with a non-reserved (or grouped) nick to become always-on.
(This can't happen with `/NS SET always-on true` because we check in
advance.)