Revert "Revert "Merge branch 'master' of https://github.com/discourse/discourse""

This reverts commit 20780a1eee.

* SECURITY: re-adds accidentally reverted commit:
  03d26cd6: ensure embed_url contains valid http(s) uri
* when the merge commit e62a85cf was reverted, git chose the 2660c2e2 parent to land on
  instead of the 03d26cd6 parent (which contains security fixes)

lib/plugin/instance.rb

@@ -165,6 +165,16 @@ class Plugin::Instance
     DiscoursePluginRegistry.register_editable_group_custom_field(field, self)
   end
 
+  # Request a new size for topic thumbnails
+  # Will respect plugin enabled setting is enabled
+  # Size should be an array with two elements [max_width, max_height]
+  def register_topic_thumbnail_size(size)
+    if !(size.kind_of?(Array) && size.length == 2)
+      raise ArgumentError.new("Topic thumbnail dimension is not valid")
+    end
+    DiscoursePluginRegistry.register_topic_thumbnail_size(size, self)
+  end
+
   def custom_avatar_column(column)
     reloadable_patch do |plugin|
       AvatarLookup.lookup_columns << column