form to upload a plugin jar in plugin tab

This is the initial commit for "enable plugin install/uninstall on the UI" feature. Work in Progress. Please feel free to give us feedback along the way. 
I think this could be problematic. If you look at the [Gemfile](https://github.com/gocd/gocd/blob/master/server/webapp/WEB-INF/rails.new/Gemfile), there are a couple of locally vendored gems, which will get lost, with this change. It's a little complicated, because Go is not purely a Rails project. The fact that it uses JRuby and is packaged as a WAR file makes it a little weird. So, bundler is not used directly at runtime, as with most Rails projects.