feat: automatically use X-Forwarded-For from built-in nginx

[nijel]

Proposed changes

In the SSL setup the X-Forwarded-For header is created by the built-in nginx server so Weblate should use it unless configured otherwise.

See WeblateOrg/weblate#13325

Checklist

• Lint and unit tests pass locally with my changes.
• I have added tests that prove my fix is effective or that my feature works.
• I have added documentation to describe my feature.
• I have squashed my commits into logic units.
• I have described the changes in the commit messages.

Other information