Use Bootstrap everywhere on the website

Recently, we’ve used [Bootstrap](https://getbootstrap.com/) on pages where we needed more flexibility on how to structure the content on the page, like:

• https://tails.boum.org/install/debian/usb-overview/
• https://tails.boum.org/donate/

Bootstrap made this extremelly easy and is still the most popular HTML+CSS framework/library.

There has been some work upstream on making ikiwiki work better with Bootstrap:

• https://ikiwiki.info/todo/merge_bootstrap_branch/
• https://ikiwiki.info/plugins/contrib/bootstrap/

Tor is using Bootstrap for their dynamic style guide:

• https://styleguide.torproject.org/
• http://oniongit.eu/infra/styleguide.git

Subtasks

• #11821

Related issues

• Related to #15800
• Related to #9216 (closed)

Original created by @sajolida on 15749 (Redmine)