Correct hyperlinks with custom url_prefix applied

[onyx-flame]

Templates and scripts had invalid links during running behind proxy with non-empty url_prefix.
It resulted in missing Celery icon in the navbar and invalid task links located in the workers table footer.
Now <a> tag of favicon takes into account prepended url_prefix and JS-generated links have a missing url_prefix()

Before:

After:

[onyx-flame]

I've updated stale minimal nginx configuration sample from examples folder.
Also now documentation describes the case with Flower placed under custom nginx location.

[mher]

Thanks for the pull request! Can you add some unit tests?

[onyx-flame]

There already was a test class which covers this case, so I've updated it a bit and simplified tests run command