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

closes #3718
- Really tiny helper returning the correct subdirectories (/ghost/ vs
/somesub/that/the/user/specified/ghost/))
- {{gh-path}} or {{gh-path ‘ghost’}} for Ghost’s root (mysubdir/)
- {{gh-path ‘admin’}} for Ghost’s admin root (/mysubdir/ghost)
This logic is already available via https://github.com/TryGhost/Ghost/blob/master/core/client/utils/ghost-paths.js.  I'm definitely for having a custom helper for this, we should just import that func here, and reuse the same logic (so that we have a single place to correct/fix it).