1904: Make Keep file EPERM test stricter.

Refs #1904.  Tom said that we want the security benefits of a 404
result, so make sure that's exactly what we get.
2753: Properly close entire Collection file tree in Workbench.
2764: Reset browser session between Workbench integration tests.

Peter and I just debugged an issue where a test was falsely passing
because the session had authorization state in it.  This prevents it
from passing.
2765: Tear out general Workbench reader_tokens support.

Refs #2765.  This approach is not tenable in its current form, and the
callbacks are making further development confusing, so I'm getting rid
of them.

Conflicts:
	apps/workbench/test/functional/collections_controller_test.rb
2764: Introduce show_file_links route.

Right now this just takes advantage of the provided reader token, but
we're going to extend it to provide a wget-friendly view.
2764: Add wget-friendly Collections file page.

This new route will become the way you share authless Collection links
with others.  They can pass it to `wget -r` to download the whole
collection, nicely organized, with nothing extraneous.  Since it
doesn't try to load user information or look up related Arvados items,
it can be rendered using an API token with a very narrow scope.

Because wget respects robots.txt, this branch stops using that in
favor of the corresponding <meta> tag.  The new view only limits
indexing, so wget can follow the links on the page.

Refs #2764.
2764: Fixed scopes_allow_request to use request.request_method (the effective
HTTP method) instead of request.method (the actual HTTP method) because
workbench uses POST even for GET.
2764: Fixed active_all_collections scoped token to be able to access
keep_disks.  Also some minor code cleanup in FileStreamer.
2764: UI overhaul to the Collection download page.

Refs #2764, based on feedback.
Merge branch '2764-wget-collections'

Closes #2764, #2774, #2830.