Migrate plugin to use theme manager

[timbrown5]

There is a new Jenkins project (which it is still in incubation, but seems to be progressing nicely) called theme manager (https://github.com/jenkinsci/theme-manager-plugin). It allows themes to be installed as plugins - not just as external CSS. I am very interested in this because of the security worried with Stylish - having s native theme seems much less of an issue. It also allows users to pick their own themes.

I have a project I started here to do this https://github.com/timbrown5/jenkins-material-theme-plugin.
I am happy to continue if you are busy with other things, but I am also happy for you to use this as a base and manage this yourself if you prefer.

I will continue trying to get to the point where the plugin is hosted on the Jenkins website unless I hear differently.