Use | for combining VariableMatches predicates

The consensus amongst the dev team was that `|` is a more intuitive
operator than `+`.

Also added unit tests and updated the flow to use this style.
Use the non-jQuery version of jointjs

The standard distribution of jointjs includes jQuery, backbone and
several other libraries.

This version excludes jQuery as we already have it (loaded from static).
I changed the layout to use the govuk header_footer_only template, so
that the standard assets (including jQuery) would be loaded.

This file was downloaded from:

https://github.com/DavidDurman/joint/tree/master/dist
Fix blank attribute value bug in Chrome