#1074 - Rebuilding discussion libraries should be able to handle `null` values
#1074 - Handle null values when getting multiple content items
Documented `404` when adding/removing content from a folder
Merge branch 'library-rebuilding-dirty-authz' of https://github.com/simong/Hilary into library-rebuilding-dirty-authz
Mixpanel tracking

Added a simple mixpanel module that can keep track of user-generated events.
This initial implementation added some tracking for content events.
Mixpanel - Authentication events

Added tracking for
 * Log in
 * Log out
 * Impostering
Mixpanel - Discussions

Tracking the following events:
 * Creating a discussion
 * Updating a discussion
 * Deleting a discussion
 * Sharing a discussion
 * Posting a discussion comment
 * Deleting a discussion comment
Mixpanel - Folders