[JENKINS-20164] Use dropdown for strategy instead of hetero-radio

[JENKINS-20164] (https://issues.jenkins-ci.org/browse/JENKINS-20164)

Jenkins introduces [structured form submission] (https://wiki.jenkins-ci.org/display/JENKINS/Structured+Form+Submission) and allows fields in each plugin not to conflict with each other.
But radio buttons can conflict with other plugins for the limitation of web browsers.
This change uses dropdown instread of hetero-radio for strategy field.

Also see jenkinsci/authorize-project-plugin#6 for details.

I upgraded target Jenkins version to 1.466, as rendering a help containing helps of dropdown items requires jenkinsci/jenkins@ab525c8c4931fa2d299d276ea1561a9b6f8a4af1.