Remove "custom on success" logic from Image.

Create a new message element and message layout element type for mod badges.

src/messages/Image.cpp

@@ -349,9 +349,6 @@ void Image::actuallyLoad()
             if (!shared)
                 return Failure;
 
-            if (shared->customOnSuccess_)
-                return shared->customOnSuccess_(shared, result);
-
             auto data = result.getData();
 
             // const cast since we are only reading from it
@@ -396,11 +393,4 @@ bool Image::operator!=(const Image &other) const
     return !this->operator==(other);
 }
 
-ImagePtr Image::setCustomOnSuccess(
-    std::function<Outcome(ImagePtr, NetworkResult)> customOnSuccess)
-{
-    this->customOnSuccess_ = customOnSuccess;
-    return shared_from_this();
-}
-
 }  // namespace chatterino