Table content removal

Refs #494.
E124
E123
Decouple feincms_nav and siblings_along_path_to from the Page model

Refs #487.
Duck typing in feincms_breadcrumbs
Import the improved queryset_transform.py file from Towel
The page manager can be a tree manager in its own right
Add a comment regarding __subclasses__
E128-clean example
Consistency is good. Start removing E128 warnings.
-E128 on contents and management
-E128, only tests and template tags missing now
E128: Only tests left
flake8 clean
Go through the list of changes, update the relnotes, add a JS comment
Order release notes chronologically
Remove autodoc extension -- it never worked correctly everywhere ever
Add a new page_is_active tag which also understands page pretenders

Thanks to Simon Bächler for the report and Bojan Mihelac for a working tag in feincmsext.

Fixes #493.
Use the central download cache of pip
Newer Pillow version in tox.ini, maybe that helps