conversation-list: adjust conversation when non-interesting mails added

This is a follow-up to !216 (merged).

When older mails appear through load_full_email(), it makes conversation to jump up and down. Fix this by adjusting the scroll.

The code is taken straight from load_conversation() → finish_loading(), but I don't deduplicate it to a separate function, because get_adjustment() isn't fast, so finish_loading() only calls it once for the whole bunch of messages.