Don't HTML-escape the assetPath with Mustache

Use Mustache's 3 curly braces to indicate that we don't want the
assetPath variable to be HTML escaped.

The assetPath is referenced from inside the href attribute of link
tags, and is generally formed of either a path or a URL containing
a hostname.

Currently, the template outputs HTML like this:

    <link href="https:&#x2F;&#x2F;foo.service.gov.uk&#x2F;css&#x2F;stylesheets/govuk-template.css" />

This commit changes it to something more like this:

    <link href="https://foo.service.gov.uk/css/stylesheets/govuk-template.css" />
Merge pull request #82 from alphagov/html-escaping-of-assetpath

Don't HTML-escape the assetPath with Mustache