[WIP] Implement signup in Ember

Closes #2410

- Add signup action that posts to signup endpoint
- Fix problem with doSignup not setting req.session.user properly (separated into it's own PR #2766)
- Automatically signin after register instead of redirecting to login
- Fix nav bar showing on signup page
- Fix image link when a user hasn't set their image yet
- Redirect to the ember/signin page if requesting an ember page

Currently based on the signin work, can rebase that out after #2764 is merged and remove the [WIP].