new-day-notifier refinement

[defanor]

Currently circe-new-day-notifier leads to mode line notifications from
every circe buffer, while it shouldn't do that at all, and it would be
nicer if it would use a different face (a "system" face or something)
for the day change messages.

Apparently the former can be solved by adding
circe-new-day-notifier-format-message into
circe-format-not-tracked, but the latter will require to modify
circe-display, or maybe to add a circe-display-something function.

[defanor]

The former can also be solved by only printing the date just before
the first message on that date: otherwise it'd be awkward for the
buffers that are inactive for weeks: just date change messages, moving
the last conversation further away each day.

Though sometimes it is a good thing to see what was the date when such
a conversation ended, without searching backwards (or looking up in a
log, even, if it exists). Perhaps it should only print an
"end-of-the-day" message if there were conversations during that day.

[defanor]

Another option is to update time marks, though that might be tricky to
implement (probably along the lines of log DB and search): something
like hour:minute → day hour → month/day → year.month, using
two-digit values and preserving the same width.

[jorgenschaefer]

If you use -server- in the format name, the format will receive a server face. That should solve the face thing.

[defanor]

But then it will look like a server message, what would be misleading.

[defanor]

Actually, "Connecting"/"Disconnected" messages also look like server messages, which is probably not quite right. But they are related to a server (a server connection), at least.