Don't expand non-interesting mails
This is implementation of a workaround suggested here #396 (closed)
When mails weren't loaded from database, conversation list may happen to
load mails through
load_full_email function, which expands mails by
Ideally this function shouldn't be called for these mails, but correct solution would require to store conversation in database, for now lets work around it by simply checking whether a mail needs to be expanded.