[systemd] Do not try to restart messageserver when there are no accounts...

.... Fixes: JB#12100

Split off the accounts check to its own .service file so we do not try
to restart messageserver several times. Also now the accounts check works
and when there are no accounts configured we do not start the messageserver.

Signed-off-by: Philippe De Swert <philippe.deswert@jollamobile.com>