Save OAuth tokens

We need that for #2809 if we're going to show suggestions about who to tip based on elsewhere data.

I've included the simplate I used to check that it works for Twitter and GitHub. Still need to confirm that it works with the other OAuth platforms (Bitbucket, Facebook, Google, OpenStreetMap and Venmo), feel free to help (especially with Venmo since I can't test that).
Okay, I'll look into it.