Allow to disable the Unsafe Browser in the Greeter

The Unsafe Browser can be used by attackers which are able to execute code as amnesia to deanonymize the user by retrieving the public IP address (#15635).

This is a low effort solution to allow users who are concerned about this to protect themselves, without hurting UX for other users.

Feature Branch: bugfix/17085-allow-to-disable-unsafe-browser

Related issues

Related to #15635

Original created by @segfault on 17085 (Redmine)

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