Re-enable social auth

For #114.
Handle existing account better

* Raise the right exception when someone tries to sign in from
  an unassociated auth provider using an email that already has
  a PyCon account
* Use the social_auth middleware that catches such exceptions, goes
  to a page you've chosen, and displays a useful message.
Don't overwrite User.email on association

When I associated my yahoo account, it changed my email to
my Yahoo email in User.  Add a django-social-auth setting to
tell it not to do that.
Merge branch 'feature/re-enable-social-auth' into develop