Add createCampaignSpec/createChangesetSpec mutations
Created by: mrnugget
The next PR to be merged into https://github.com/sourcegraph/sourcegraph/pull/11675.
This adds more data-layer infrastructure: the createCampaignSpec
and createChangesetSpec
mutations. There are still TODOs, the big ones revolving around validation of the specs, but since we haven't settled on a schema yet, I just left them in there.
The two mutations also check for repository permissions.