coretasks: no realname for new JOINs on away-notify servers

[dgw]

As far as I can tell, the only place the realname of a newly joined user might be fetched is in the periodic WHO requests Sopel sends, which are currently skipped if the server supports away-notify:

sopel/sopel/coretasks.py

Lines 806 to 808 in 9965cce

	if 'away-notify' in bot.enabled_capabilities:
	# WHO not needed to update 'away' status
	return

My half-baked ideas for this (not a ton of free time to think/research today) are:

• Skip the above check and always _send_who() for channels periodically
• Immediately WHO the exact nick after a user JOINs a channel and hasn't been seen before

I will edit the list if I come up with more ideas when actually working on this.

Follow-up to: #2383