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. 
Take a look at [tests for this](https://github.com/gocd/gocd/blob/master/server/webapp/WEB-INF/rails.new/spec/views/admin/plugins/plugins/index_html_spec.rb).