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.
Oops, just responded to your previous comment without seeing this one. Yeah, it's not really necessary in `get_response` as long as we can queue it perhaps via the command line. I could just run `rails console` and then `PrayerLettersOAuthUpgrader.perform_async` which does make things clearer. I'll plan to go that way instead unless you suggest otherwise and then yes, I can take it out of `get_response`