Will-fix: #10785

It broke too: https://jenkins.tails.boum.org/job/test_Tails_ISO_stable/78/

refs: #10774

In Jessie's GNOME we don't have arrows on the scrollbars, so it's hard
to find a good image identifying the right part to click to scroll
up. Luckily Page Up/Down are mapped to scrolling the chat log, so we
can use that instead.

Have the security advisory against 1.7 point to 1.8.1, since 1.8 doesn't fix all issues fixed in here.

On https://jenkins.tails.boum.org/view/RM/job/test_Tails_ISO_experimental/67/
the screenshot shows that Evince did manage to open it, but the video lacks
the frames that prove it ⇒ I bet that Evince needed just a tiny bit more
than 10 seconds.

refs: #10776