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 default.

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.