Re-open current message when leaving offline mode

When user opens a message and get's the "You are offline" or "Account
is offline" message, they can click the "Go online" link. Upon clicking,
instead of showing them the KMail Welcome page, we now re-load the message
so that it is retrieved from the backend and displayed right away.

Differential Revision: https://phabricator.kde.org/D7547