Introduced a QueueItemAuthenticator overload taking Task.

And moving the permissions checks in BuildTrigger from being hardcoded in execute to being overridable in Dependency.
This has two benefits:
· We can restore the previous form validation logic predicting whether Item.BUILD will be available.
· A (true) Trigger could check Item.READ on the upstream project
  under the authentication that the downstream project would have if it were to be built.
  (This part is not yet implemented; probably will want to make this replace the current pseudotriggers.)