[REF] Support endpoint-based URL generation in UH
[IMP] Support url generation in JS
[REF] Use build_url when creating URLs in JS

- only used for urls defined in Indico.Urls
- there may be others passed e.g. through a template
- those need to be fixed later when switching to clean urls
- trying to build incomplete urls will fail with a BuildError so they
  will be easy to spot