Move post slug endpoint & add endpoints for users

closes #3187

- move slug endpoint to post/slug/:slug
- create similar  slug and email endpoint for users
- add/update tests
:+1: the regex filter was too restrictive before.  When neither of those endpoints matched, Express returned the catch-all route (i.e., default.hbs) with a status code of `200`.  Really confusing to both people and Ember Data.