4408: Workbench Collection sharing allows file downloads again.

Earlier we refactored our API token loading code to load User.current
to check that it was valid.  This approach doesn't work when we're
presenting shared Collections, because the token usually won't be
scoped to get user information.  Skip the check for this specific
case.
Merge branch '4408-collection-sharing-login-fix-wip'

Closes #4408.