Remove request.fs

[chadwhitacre]

We stopped using it in #389, but failed to in fact remove it. Closes #450.

[chadwhitacre]

Ready for review, @techtonik @pjz.

[techtonik]

The explanation what should be used instead is still missing from
https://github.com/gratipay/aspen-python/blame/master/Changelog.md#L41
Without it I am unable to fix gratipay/inside.gratipay.com#263 (comment) so for me the issue won't gone.

[chadwhitacre]

Reticketed #453.

[techtonik]

@whit537 is it related?

[chadwhitacre]

@techtonik Yes. That's where we document that Aspen uses algorithm.py, which is what you need to know to understand add_nav_current_to_context.

[techtonik]

@whit537 I understand that this nav_current needs the filename of the current simplate to calculate navigation, but I have no idea where to get one.

[techtonik]

Ok. The issues seem non-trivial, so let's upgrade, let it fail with new traceback and file a new bug report.