WebJar CDN substitution is only performed where the user declares a path fallback in their main configuration. This should be more reliable in general, and faster given that not every js file in a WebJar has its path declared up front (which causes rjs to process it).

Removed build writer (yay!) by building the paths a different way. This also facilitates the ease in which the user can extend paths by just using:

RjsKeys.paths += "someModule" -> "empty:"