Traverse to theme resources from the portal

[pbauer]

Fix broken theme when rendering accessible content contained in an inaccessible navigation-root.
Fixes #142

[mister-roboto]

@pbauer thanks for creating this Pull Request and helping to improve Plone!

TL;DR: Finish pushing changes, pass all other checks, then paste a comment:

@jenkins-plone-org please run jobs

To ensure that these changes do not break other parts of Plone, the Plone test suite matrix needs to pass, but it takes 30-60 min. Other CI checks are usually much faster and the Plone Jenkins resources are limited, so when done pushing changes and all other checks pass either start all Jenkins PR jobs yourself, or simply add the comment above in this PR to start all the jobs automatically.

Happy hacking!

[pbauer]

@jenkins-plone-org please run jobs

[yurj]

LTGM

We should add some docs to explain how theme resources traversing works.

[MrTango]

This might make problems for sub sites like collective.lineage, did you test that?

[pbauer]

I don't think this is a issue since subsites that use a different theme with (using lineage.themeselection) should still have normal theme-files that are traversable from the site-root. I've never had that use-case and did not test it though.

[mauritsvanrees]

This causes a regression: #236

I have a proposed fix here: #237
Please check if your use case still works with that code.