Allow to disable the Unsafe Browser in the Greeter

Originally created by @segfault on #17085 (Redmine)

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 (closed)).

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 (closed)

Edited May 15, 2020 by segfault

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