Add `swagger validate` to drone run

[stevegt]

Once we get the swagger validation errors cleaned up in #4010, we'll want to add a test case to keep them that way:

swagger validate public/swagger.v1.json

Background: Various swagger clients try to validate at startup; when validation breaks, the clients break. In my own case, we need working clients to be able to migrate to gitea from legacy systems, and we'll continue to need them for daily ops after that.