Stop using govuk mustache template module

[joelanman]

We pull in both the Jinja and mustache template, and use the assets from mustache. We could just use the assets from Jinja. Is there any downside, taking into account Jinja is a repo, not an npm module

[gemmaleigh]

At the moment, we're using the packaged release of the Jinja version of the govuk_template.

It would mean we wouldn't be able to use npm outdated to find out when there is a new version of the govuk_template, but we're likely to know when it is updated.

Ideally, a new repo would be set up for the Jinja version the govuk_template.

What about:

• raise an issue with the template to publish the Jinja version to npm (cc. @dsingleton, @alexmuller).
• remove the grunt tasks which copy assets from the Mustache version of the template and instead copy the assets from the Jinja version
• when the Jinja version of the template is published as an npm module, update package.json