Add "Don't Show Again" button to notifications where appropriate

There are notifications that a given user will basically see every time they start Tails in their usual context. For those, it would be nice if the user could opt-in to a persistence preset that allows them to check “don’t ask me again”. If we had this, we could also dare making these notifications more noisy, e.g. using other technologies than desktop notifications.

Where to use this feature

By decreasing order of frequency:

1. Synchronizing the system’s clock (#7439 (closed))

   We might as well remove it all the way.

   But we could add a "Don't Show Again" until we get there in 6 months or so.

2. Wi-Fi interface doesn't work (#14534)

   We don't have this notification yet but having "Don't Show Again" would pave the way.

   It would be good to tailor this one to the Wi-Fi interface.

3. Installing Additional Software

   

4. Warning when starting Tails in a VM (!284 (merged))

   So far we advertize ISO images for virtual machines so we won't be able to store this information anyway.

   For now, this could be useful for the ¼ of virtual machines users who already run on a vfat file system.

   That's probably around 2% of our current user base.

   In the future, we could distribute USB images for virtual machines. (#18054)

5. MAC Spoofing failed

   Very rare.

Where NOT to use this feature

• Instruct about the possibility of creating a Persistent Storage when there is none (#15586 (closed))

  We should instead:

  • Use the space of the Persistent Storage in the Welcome Screen to advertise it, as per #15586 (closed).
  • Display a fancy splash on first boot, but this doesn't require "Don't Show Again". We could use "Don't Show Again" ask to display this splash every time until the user chooses "Don't Show Again" but we would have to store this bit of information outside of the Persistent Storage, so let's skip this debate for now 😉

• This version of Tails has known security issues

  I'd rather always display this one.

• Additional Software are installed or failed installing

  Users need to know when Additional Software is ready or broken.

• Warning when starting Unsafe Browser

  The Unsafe Browser is too dangerous to allow people to skip this one.

• Tor is not ready when starting Tor Browser (#8061)

  I don't see what we gain in allowing people to skip this one.

• Propose users to add relevant internationalization packages (#16611)

  We will have a better time adding this as a permanent option somewhere in the Persistent Storage settings.

Design

In all cases of GNOME notifications, we could add 2 buttons:

`[Don't Show Again]` and `[Close]`

Otherwise too many people are going to click through [Close].