Small weirdness in foreignKey handling

Description

While working on #5799 I saw that there are some small weird things in foreignKey handling which might slow things down a bit

First one is some object types appear both as uppercase and lowercase in the $foreignRefs temp array instead of appearing only once (should be easy to fix).

Second one is that ACL assignment foreign keys appear both as from ACL assignment object type and each departement object type, which duplicate the LDAP search (harder to fix, might need a hardcoded special case for this tab).

FusionDirectory Version

1.3-dev

Steps to Reproduce

1. Add a debug dump for $foreignKey temp array
2. Go to the references tab of a user
3. Look at the dump

Expected behavior:

Clean well-organized array

Actual behavior:

Some duplicated info and mixed upper/lower case for object types