Ensure notifications are cleared on app activation

Clear both new mail and send mail error notifications on activate, since
in the case of the former the new mail will be selected, and in the
case of the latter an error info bar will be displayed in the main
window.
5 jobs for <span class="ref-name">wip/766133-gnotification</span> in 22 minutes and 58 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #274429
fedora

00:06:27

passed #274430
ubuntu

00:07:33

 
  Test
passed #274431
flatpak

00:14:21

 
  Deploy
passed #274432
review

00:00:30

passed #274433
allowed to fail manual
stop_review

00:00:33