{{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)