Reviewed PR to add import user avatars va LDAP

- Reduced options to single new configuration paramter instead of two.
- Moved more logic into UserAvatars class.
- Updated LDAP avatar import to also run on login when no image is
  currently set.
- Added thumbnail fetching to search requests.
- Added testing to cover.

Related to PR #2320, and issue #1161

app/Config/app.php

@@ -184,7 +184,6 @@ return [
 
         // Custom BookStack
         'Activity' => BookStack\Facades\Activity::class,
-        'Images'   => BookStack\Facades\Images::class,
         'Permissions' => BookStack\Facades\Permissions::class,
         'Theme'    => BookStack\Facades\Theme::class,
     ],

app/Config/services.php

@@ -133,8 +133,7 @@ return [
         'remove_from_groups' => env('LDAP_REMOVE_FROM_GROUPS', false),
         'tls_insecure' => env('LDAP_TLS_INSECURE', false),
         'start_tls' => env('LDAP_START_TLS', false),
-        'import_thumbnail_photos' => env('LDAP_IMPORT_THUMBNAIL_PHOTOS', false),
-        'thumbnail_attribute' => env('LDAP_THUMBNAIL_ATTRIBUTE', 'thumbnailPhoto'),
+        'thumbnail_attribute' => env('LDAP_THUMBNAIL_ATTRIBUTE', null),
     ],
 
 ];