Make font-end libraries customisable

Currently, we ship a bunch of frontend libraries (such as jquery). With frontend libraries that include css, such as bootstrap and pure it becomes necessary to be able to customise the CSS and run it via a preprocessor again. We need to start making use of that whole ecosystem of tools (npm, bower, grunt, less, rework, browserify, etc) to keep track of versions of our libraries, and to allow users to supply custom builds of these libraries. Perhaps a switch can also be built in here to fetch some of these things via a CDN rather than our shipped versions.