Allow full URLs to be used as the avatar path

This is useful for forums integrating with an external website (eg. a
WordPress site), so they can reference existing avatars directly.

For alternative storage locations (eg. S3) the best practice will still
be to store a relative path and then configure an external base "assets
URL" (this is not currently possible - TODO).

Given this change, I think it would probably make sense to rename the
column to `avatar_url` in the upcoming batch of database naming changes
- then it can contain either a relative or an absolute URL -
@franzliedke do you agree?

framework/core/src/Core/User.php

@@ -323,7 +323,14 @@ class User extends AbstractModel
      */
     public function getAvatarUrlAttribute()
     {
-        $urlGenerator = app('Flarum\Forum\UrlGenerator');
+        if ($this->avatar_path) {
+            if (strpos($this->avatar_path, '://') !== false) {
+                return $this->avatar_path;
+            } else {
+                return app('Flarum\Forum\UrlGenerator')->toPath('assets/avatars/'.$this->avatar_path);
+            }
+        }
+    }
 
         return $this->avatar_path ? $urlGenerator->toPath('assets/avatars/'.$this->avatar_path) : null;
     }