Have a test website on staging.tails.boum.org

Originally created by @sajolida on #9183 (Redmine)

This will allow us to:

• Develop web prototypes without polluting our main repo.
• Have web prototypes available for review or testing online.
• Allow more people to push changes through Git.

Subtasks

• #9184 (closed)
• #9185 (closed)
• tails#9186 (closed)
• #9187 (closed)