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.
Definitely makes sense to get the OAuth1 stuff out. Steve from PrayerLetters suggested a multi-part process: 1) deploy code that will use OAuth2 but fall back to OAuth1 when needed, 2) run the upgrade script to upgrade everyone to OAuth2, 3) verify that everyone is upgraded to OAuth2, then 4) remove the OAuth1 code. Does that sound like a good process to you? Or would you want to basically skip the step of deploying the OAuth2/OAuth1 fallback code?