[web/test] Drop folder cube dependency
Copy useful schema bits, define IBreadCrumbs adapter for Folder/fild_under
(previously relying on ITree adapter) and adjust unittest_viewselector tests
to drop a view that does not exist anymore.
Related to #5168939.
What's new in CubicWeb 3.21?
============================
Code movement
-------------
The cubicweb.web.views.timeline module (providing the timeline-json, timeline
and static-timeline views) has moved to a standalone cube_
.. _cube: https://www.cubicweb.org/project/cubicweb-timeline
Deprecated code drops
---------------------
* the user_callback api has been removed; people should use plain
ajax functions instead
* the `Pyro` and `Zmq-pickle` remote repository access methods have
been entirely removed (emerging alternatives such as rqlcontroller
and cwclientlib should be used instead). Note that as a side effect,
"repository-only" instances (i.e. without a http component) are no
longer possible. If you have any such instances, you will need to
rename the configuration file from repository.conf to all-in-one.conf
and run ``cubicweb-ctl upgrade`` to update it.