app.rs: Update the last viewed message

This updates the last viewed message when the user goes into
a room.

Closes https://gitlab.gnome.org/danigm/fractal/issues/75
1 job for mark-last-viewed-message in 5 minutes and 48 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Test
passed #12290
test:stable

00:05:48