[Request] Add social login through OpenStreetMap

[mradamcox]

Describe the feature or enhancement you're looking for
Adding social login, through Facebook, Gmail, etc has always been a generally attractive idea, but adding OpenStreetMap would be incredible--it's the top priority in this realm of the app.

When a user creates a new account with their OSM id, this would create a new profile in the Django admin and link it to the OSM login. Beyond that, I'm not sure what all this update will require.

Ultimately, a workflow will be needed to migrate an existing account into a OSM-linked account.

Alternatively, and probably better, would be a workflow that an existing user could initiate through their profile page to link to their existing OSM account, and then login through OSM auth in the future.