Begin implementing STI for User model

- Begin implementing Single Table Inheritance for the User model so
  users can either sign in with a CNetID or sign up without one
Fix initial LDAP + DB auth bugs

- Fix initial bugs that surfaced when switching from only authenticating
  with LDAP to authenticating with both the database and LDAP
Fix user creation bug in sample_data.rake
Add registration page
Continue working on the sign up feature

- To be implemented: a separate sign in page or form for local users