Fix missing partial when editing comments with @mentions (#2828)

Edge Rails introduced ActionText::Attachable#to_editor_content_attachment_partial_path
which defaults to to_partial_path ("users/user"). Override it in User::Mentionable to
delegate to to_attachable_partial_path ("users/attachable"), matching the existing
display rendering path.

app/models/user/mentionable.rb

@@ -8,6 +8,10 @@ module User::Mentionable
     def to_attachable_partial_path
       "users/attachable"
     end
+
+    def to_editor_content_attachment_partial_path
+      to_attachable_partial_path
+    end
   end
 
   def mentioned_by(mentioner, at:)