New URL helper - URL consistency fixes

fixes #1765
fixes #1811
issue #1833

New UrlFor functions

- moved body of url helper to config.path.urlFor, which can generate a URL for various scenarios
- urlFor can take a string (name) or object (relativeUrl: '/') as the first
  argument - this is the first step towards issue #1833
- also added config.path.urlForPost which is async and handles getting
  permalink setting
- frontend controller, ghost_head helper, cache invalidation all now use
  urlFor or urlForPost all urls should be correct and consistent

URL Consistency Improvements

- refactored invalidateCache into cacheInvalidationHeader which returns a
  promise so that url can be generated properly by urlForPost
- moved isPost from models to schema, and refactored schema to have a tables object
- deleted posts now return the whole object, not just id and slug,
  ensuring cache invalidation header can be set on delete
- frontend controller rss and archive page redirects work properly with subdirectory
- removes {{url}} helper from admin and client, and replaced with adminUrl
  helper which also uses urlFor
- in res.locals ghostRoot becomes relativeUrl, and path is removed