Made LDAP auth ID attribute configurable

- Allows the field that gets stored as the "External Authentication ID"
to be configurable. Defined as LDAP_ID_ATTRIBUTE=uid in .env.
- Added test to cover usage.
- Also now auto-lowercases when searching for attributes in LDAP
response since PHP always provides them as lower case.

Closes #592.

.env.example.complete

@@ -191,6 +191,7 @@ LDAP_PASS=false
 LDAP_USER_FILTER=false
 LDAP_VERSION=false
 LDAP_TLS_INSECURE=false
+LDAP_ID_ATTRIBUTE=uid
 LDAP_EMAIL_ATTRIBUTE=mail
 LDAP_DISPLAY_NAME_ATTRIBUTE=cn
 LDAP_FOLLOW_REFERRALS=true