Add better uid + provider unique support

[booleanbetrayal]

This may be superseded by any of the "support multiple omniauth" tickets, but currently, we treat uid as unique despite the fact that we don't set relational constraints as such (justuid +provider), via a find_by_uid call. What should be happening at this point (if we don't get the consolidated multiple provider work done) is also passing up theprovider.

[zachfeldman]

Hi there @booleanbetrayal ,

In an effort to cleanup this project and prioritize a bit, we're marking issues that haven't had any activity in a while with a "close-in-7-days" label. If we don't hear from you in about a week, we'll be closing this issue. Obviously feel free to re-open it at any time if it's the right time or this was done in error!

If you are still having the issue (especially if it's a bug report) please refer to our new Issue Template to provide some more details to help us solve it.

Hope all is well.