Managed Scripts plugin causes a javascript crash on the job configuration page on Jenkins 2.264+

I have created a PR for this issue, but there are several changes that are needed (also created PRs for them):

1. Update Jenkinsfile to build with JDK8 instead of JDK7 (and fix for Maven 3.6.0+)
2. Update repo URLs in pom.xml to https (from Daniel Beck)
3. Update plugin minimum Jenkins version and plugin docs
4. Fix the table to div regression