Adding oauth provider for submission system and enabling voting

This is my take on enabling dtsato/agile_brazil#82

Please review it and deploy if you think it's good. I've already created the oauth client_app in staging and prod on the submission system, and setup the right tokens/secrets in the config.yml for the registration system.

We should test this amongst ourselves before pushing live. Don't know how to create an "early bird" registration without changing the deadlines... and the link will only show up if the registration is confirmed (paid), which means I had to poke the DB 'cause I didn't want to pay 300 reais :)

I also created a page to describe the process of setting up for voting. Please send updates to that instruction text too if you think it's not clear.

\cc @ceci