add functionality to use freestyle templates for matrix jobs

[JeremyMarshall]

Whist you can't change a job type once it exists, what is possible is to create a new job based on a template of a different type. I did this for freestyle and matrix jobs as that seemed the most obvious.

It works by adding or removing elements and changing the type. Hopefully it is extendible to swap between other job types by writing the class to coerce it

[jenkinsadmin]

Thank you for a pull request! Please check this document for how the Jenkins project handles pull requests

[JeremyMarshall]

I did this over a year ago and don't have my repo anymore. There looks to be conflicts. Happy to bring it up to date if it is still viable and wanted

[jiff-infrastructure]

Can one of the admins verify this patch?

[CorevistCI]

Build finished.