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,
     ],