{{gh-path}} Handlebars helper, returning paths

- Uses utils/ghost-paths to resolve paths
- {{gh-path}} or {{gh-path ‘blog’}} for Ghost’s root (mysubdir/)
- {{gh-path ‘admin’}} for Ghost’s admin root (/mysubdir/ghost)
- {{gh-path ‘api’}} for Ghost’s admin root (/mysubdir/ghost/api/v0.1)
Merge pull request #3824 from felixrieseberg/iss3718-2

{{gh-path}} Handlebars helper, returning paths