Do not try to parse raw ChangesetSpecs as YAML
Created by: mrnugget
In https://github.com/sourcegraph/sourcegraph/issues/16081 we discovered
that the YAML parser in go-yaml
can choke on control characters, which
can be found in a diff.
Leaving aside whether that is a bug in the YAML parser or not, it doesn't make sense that we try to parse the raw changesetspec as YAML, since it'll never be YAML.
So, with https://github.com/sourcegraph/campaignutils/pull/7 we now have the ability to only parse and validate the input as JSON without going through the YAML parser.
That in turn fixes #16081 on the server.