Update OpenPGP verifications to Tor Browser not opening applications

Since we sandboxed Tor Browser, it’s not possible to open applications associated with downloaded files. This breaks the instructions in /doc/get/verify_the_iso_image_using_gnome (soon to be moved to /doc/advanced_topics/openpgp_verification/gnome). We should rewrite them to be Tails-proof and tell to save the file and do the verification from Nautilus instead.

The notification system of Tails 2.0 is different as well, so screenshots need to be adjusted, etc.

Still, that’s low prio as the new installation assistant won’t require people to following this as much.

Related issues

• Related to #9285 (closed)
• Is duplicate of #8040 (closed)
• Blocked by #11027 (closed)

Original created by @sajolida on 10882 (Redmine)