Show a warning in case privacy.resistFingerprinting is enabled

[rugk]

Background

The issue is #22 and this already lead to problematic reviews who attributed the problem to the add-on instead of Firefox, respectively a Firefox setting.

Proposed solution

While the above linked problem cannot be solved, we can use a WebExtension API to read that setting and show a warning or potentially even disable it with one click.

So a tip/warning should be shown for that IMHO.

In detail:

• when the toggle is used or when the settings page is opened, there should be a warning
• the toggle may just open the settings page in such a case or a separate one

Alternatives

Leave it as it is?
Document the problem in other places e.g. the Readme, AMO description etc.? However, the proposed solution is IMHO UX-wise best, because only affected users see it (no others are disturbed) and those (the majority) who do not have any problems here, just leave it as it is… 🙃

Additional context

We also already have the permission to adjust browser settings, so implementation-wise this is not a big deal.