Add handler_url usable by descriptors

[LMS-1614]
Allow multiple client-side runtimes on a single page

Make XBlock client-side runtimes proper classes, so that handlerUrl can
be defined in a per-runtime way, and we can have multiple runtimes on a
single page.

[LMS-1630][LMS-1421][LMS-1517]
Make Studio load XBlock fragment js and css on the client-side

[LMS-1421][LMS-1517]