We need to unlock the user keyring anyway to unlock the address keyring,
so we should just return it instead of re-unlocking the user keyring
when sending a message.
When an IMAP client appends a message to a mailbox, it can specify
which flags it wants the appended message to have. We need to handle
these in a proton-specific way; not-seen messages need to be imported
with the Unread bool set to true, and flagged messages need to
additionally be imported with the Starred label.
