Auth services management

Implements two pivotal stories:

"See which services I have associated with my account"
https://www.pivotaltracker.com/story/show/50623833

"Delete services from my account"
https://www.pivotaltracker.com/story/show/50623845

Note that the pull-content process will need to check for a valid UserSocialAuth before trying to pull content; this should already be the case.