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.

app/Config/services.php

@@ -123,6 +123,7 @@ return [
         'base_dn' => env('LDAP_BASE_DN', false),
         'user_filter' => env('LDAP_USER_FILTER', '(&(uid=${user}))'),
         'version' => env('LDAP_VERSION', false),
+        'id_attribute' => env('LDAP_ID_ATTRIBUTE', 'uid'),
         'email_attribute' => env('LDAP_EMAIL_ATTRIBUTE', 'mail'),
         'display_name_attribute' => env('LDAP_DISPLAY_NAME_ATTRIBUTE', 'cn'),
         'follow_referrals' => env('LDAP_FOLLOW_REFERRALS', false),