Solve catalogue authorization issues + code improvements

Fix: Catalogue doesn't show for anonymous user (only after logout-login of admin user)
Fix: MolgenisUser x with role=normal can only access StudyDataRequest for which MolgenisUser is user x
Fix: Catalogue doesn't show for registered user, because registered user does not belong to any group
Improvement: replace hard coded strings with constants
Improvement: resolve runtime c3p0 warning
Improvement: use UUIDs for some identifiers to ensure that they are globally unique