Consolidate all knowledge of whether a session is authenticated into AuthenticationUtil.

Catch all other XMLUI uses of session attribute "dspace.current.user.id"
(which is only set in JSPUI).

Factor testing of "dspace.user.effective" into a new static method to
deduplicate logic and encapsulate the meaning of this session attribute.
Merge pull request #1 from mwoodiupui/patch-1-mark

Patch 1 amended with changes from Mark Wood