Remove unused Fomantic sidebar module (#16853)

* Remove unused Fomantic sidebar module

The [Sidebar](https://fomantic-ui.com/modules/sidebar.html) module seems
currently unused (at least I can't find any reference to it in templates
or js), so remove it from the Fomantic build.

* remove useless minified fomantic build files

* mark fomantic build files as being generated

.gitattributes

@@ -4,3 +4,4 @@
 /templates/**/*.tmpl linguist-language=Handlebars
 /.eslintrc linguist-language=YAML
 /.stylelintrc linguist-language=YAML
+/web_src/fomantic/build/** linguist-generated