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. 
There's opportunity to make this a JSON-only API and use it from the view too. But, if you want the page to refresh, for now, then it's ok. As I mentioned in my earlier comment, ideally, upload would be an API