Add proofed_by field to Election model #11

* Add ProxyUser model to clean up FK lookups and default ordering in admin
* Add migration for proofed field
* Display proofed_by field on Election admin detail page in Notes section
Substitute ProxyUser for User foreign keys to improve admin readability

Long term we should really switch to a custom user and port Volunteer model.
merge proofed_by field and proxyuser updates in models.py
Merge branch 'master' of github.com:openelections/dashboard