#514 db.connection

db.connections[0] === db.connection
User Auth Refactor

Public:
- Wrapped auth http requests into it's own service
- Improved login and register templates error handling to utilize bootstrap form errors
- Removed interceptor in exchange for a better one that remembers failed 401 requests and retries them after authorization has been confirmed.

Server:
- Changed `/loggedin` to respond with appropriate http codes
- Improved user validation handling by isolating it to the model only and removing controller validation. This makes error handling on the public side more consistent.
- Added plugin to improve `unique` field handling by responding with proper validation errors. Also improves error handling consistency.
Merge branch 'master' of https://github.com/linnovate/mean

# By Drew Fyock
# Via Drew Fyock
* 'master' of https://github.com/linnovate/mean:
  #514 db.connection