Improve Ubuntu installation instructions

https://tails.boum.org/install/debian/usb/index.en.html

2/6. Install Tails Installer ▸ If you are running Ubuntu ▸ 2. Open a Root Terminal.

No Root Terminal is installed, at least not on a fresh install of Ubuntu GNOME 17.04. Or if it is installed, I think we need to explain better how to open it.

Maybe instead we should explain how to add the tails-installer PPA through the graphical Software & Updates application?

Related issues

Related to #11378 (closed)
Blocks #13423 (closed)

Original created by @cbrownstein on 12695 (Redmine)

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information