Prayer letters OAuth2 upgrade

This uses OAuth2 to connect authorize future PrayerLetters.com integrations instead of OAuth1. It also uses OAuth2 for the API requests (with OAuth1 as a fallback for users who haven't been migrated yet) and runs a background job to migrate the old accounts to OAuth2.

I also added code to log the first request exception that then causes a JSON parse exception for the error code in `PrayerLettersAccount.create_contact`. That will hopefully help in tracking the underlying cause of that error in Errbit.

This isn't ready for deployment yet as the `omniauth-prayer-letters` gem it uses is being hosted in my personal github repo, and we need the client id and secrets from PrayerLetters before I can test it out on stage or deploy it to production, but the code is pretty much ready at this point.
Yes, it would be transitionary. After everyone is upgraded, I could go back and just validate oauth2 tokens (perhaps that would work now as well, I just wanted to err on the side of caution to allow oauth1 accounts during the transition).