Replace PHP backend with pure JS

[jchristi]

The only part that was unique to PHP was twig, but there is a pure-js implementation of twig now: https://github.com/twigjs/twig.js. The REST server could easily be accomplished with Express or equivalent. A pure JS implementation would remove the dependency on PHP.

[castastrophe]

Unfortunately TwigJS has really poor Twig PHP support.

From https://github.com/twigjs/twig.js/wiki:

Twig.js is currently a work in progress and supports a limited subset of the Twig templating language (with more coming).

[jchristi]

https://github.com/twigjs/twig.js/wiki/Implementation-Notes

61/67 features are supported