Use BASIC authentication so that an API token may be specified

Necessary in order to publish to a public server using a security realm not based on `AbstractPasswordBasedSecurityRealm` or its kin; discovered while trying to publish to a *.ci.cloudbees.com host.

One downside is that if someone is currently publishing to a password-based security realm, they will need to switch to their API token after an upgrade in order to continue to authenticate. They ought to do so anyway—it is simpler and more secure—but if this is cause for complaint, it should be possible to keep the existing `password` field and add a separate `apiToken` field, explaining in the UI why you should prefer to migrate to the API token, and use either method of authentication as configured.