3072: add an empty collection in db seeds

3072: add an empty collection in db seeds

Added empty collection (owned by the anonymous group) to test fixtures
and to db/seeds.rb.

API integration tests: in permissions_test.rb, test that the active user
can read objects in the anonymous group, even when there are no explicit
permission links on that group or objects in it.

Workbench integration tests: added a test to confirm that a non-admin
user can get the empty collection.

Cleanup code: User.can? now takes either a uuid or an object for the target.

Refs #3072.