FIX: Issues with custom icons in themes (#13732)

Fixes two issues:
- ignores invalid XML in custom icon sprite SVG file (and outputs an error if sprite was uploaded via admin UI)
- clears SVG sprite cache when deleting an `icons-sprite` upload in a theme

spec/models/theme_field_spec.rb

@@ -419,6 +419,17 @@ HTML
       theme_field.update(upload: Fabricate(:upload))
       expect(theme_field.value_baked).to eq(nil)
     end
+
+    it "clears SVG sprite cache when upload is deleted" do
+      fname = "custom-theme-icon-sprite.svg"
+      sprite = UploadCreator.new(file_from_fixtures(fname), fname, for_theme: true).create_for(-1)
+
+      theme_field.update(upload: sprite)
+      expect(SvgSprite.custom_svg_sprites(theme.id).size).to eq(1)
+
+      theme_field.destroy!
+      expect(SvgSprite.custom_svg_sprites(theme.id).size).to eq(0)
+    end
   end
 
 end