[FIXED JENKINS-22550] Properly delegate to web UI of TimerTrigger.

[JENKINS-22550](https://issues.jenkins-ci.org/browse/JENKINS-22550): incompatibility in Jenkins core only matters if you are directly calling `@WebMethod`s from outside, whereas the intended form of reuse is as URLs. Restores compatibility with cores before and after https://github.com/jenkinsci/jenkins/commit/f97b5b33b0754532b945340cdee4b6ea1d2f6ba4; offers more advanced UI when using the newer core; shows help; and also switches to hash-balanced scheduled by default.

You may also want to switch to `f:textarea`, which is what `Trigger.spec` normally is shown as.