campaigns: restore pre-3.20 instance compatibility
Created by: LawnGnome
The author name and e-mail fields were always sent, but the schema Sourcegraph 3.19 uses to validate changeset specs don't include these fields. Since we're doing version detection for feature flags (quirks) now anyway, let's use that to not set the author name and e-mail if they're not explicitly provided when talking to a 3.19 instance.
If the user provides an author name and/or e-mail and sends a changeset spec to a pre-3.20 instance, the request will fail, but since those are 3.20 features that's OK.
