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.
This wasn't super clear, but the PrayerLettersOAuthUpgrader actually does upgrade everyone, but it gets queued the first time anyone with OAuth1 does any sync operation (it's also a unique job so if it's running it won't get queued again). Perhaps I should have just run it directly on the machine, but I figured launching it from the code would eliminate the need to SSH in and figure out how to queue it well, but we could go that routes as well if you think it's better.