[WiP] [JENKINS-16956] Enforce build trigger security

[JENKINS-16956](https://issues.jenkins-ci.org/browse/JENKINS-16956): ensure that the user associated a build (or the anonymous user, if unspecified) is able to see and build the downstream project.

Supersedes the placeholder fix of SECURITY-55 (and SECURITY-109): rather than checking permissions at job configuration time, rely on `QueueItemAuthenticator`s (such as that in the `authorize-project` plugin) to authorize us.

TBD what the migration behavior should be for installations which currently lack a QIA. For example, if you deny anonymous read access, and have no QIA, then with this patch no downstream builds will ever be triggered. Which is actually correct, after a fashion, but would be seen as a regression.