LDAP: fix potential infinite loop introduced with 4c4aa92e, as side effect optimize groupExists-method.

Fix password authentication for links and fix template problems for links by creating a new base layout

LDAP: check for existing username from other backends when creating one for an LDAP user or group. Led also to some code cleanup. Fixes oc-1551 in master.

Check usernames in the database as case insensitive, they are still stored case sensitive. Bug fix for oc-422 and oc-1514