Add support for browsing as anonymous user
When the config says:
AllowAnonymousUserAccess: trueWorkbench2 should:
- have a prominent "Browse public projects" link, even when ther user is not logged in
- add the anonymous user to the "share with" options when objects are shared
- add the anonymous group to the "share with" options when objects are shared
AllowAnonymousUserAccess is false, it should do none of these things.