FEATURE: support custom icons in themes (#7155)

* First take

* Add support for sprites in themes

Automatically register any custom icons added via themes or plugins

* Fix theme sprite caching

* Simplify test

* Update lib/svg_sprite/svg_sprite.rb

Co-Authored-By: pmusaraj <pmusaraj@gmail.com>

* Fix /svg-sprite/search request

lib/upload_creator.rb

@@ -37,6 +37,7 @@ class UploadCreator
 
       is_image = FileHelper.is_supported_image?(@filename)
       is_image ||= @image_info && FileHelper.is_supported_image?("test.#{@image_info.type}")
+      is_image = false if @opts[:for_theme]
 
       if is_image
         extract_image_info!