Better user mapping

GitHub users that choose not to publicize their membership in an organization will not have their email available to us. Therefore we need to choose something else to map from their OAuth identity to our internal User model. This PR adds a `uid` column to the `User` model which will store either the id number from a Google OAuth `identity_url` or the `uid` value from GitHub OAuth.

After performing the upgrade you'll need to [migrate](https://github.com/obfuscurity/descartes#migrations) (`bundle exec rake db:migrate:up`) and then restart the service again to load the new User model. In the off chance that you're already running the new User model code (#87), you will probably want to delete your users (`delete from users;` or `VERSION=6 bundle exec rake db:migrate:to && bundle exec rake db:migrate:up`) before performing the aforementioned migration.