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.