Remove the use of Rework in the browser, add server-side Rework endpoint

This stops trying to use Rework in the browser which really isn't support. See reworkcss/rework#152

A new endpoint, `/start/css`, has been added which dynamically generates custom Pure Grids CSS based on the query. This is used by the Pure Starter Kit client-side app to update its output view.

Overall this resulted in code removal!

Closes #221