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. 
From what it's doing right now, yes it would. Do you think it would cause some wonky-ness if we created a duplicate? i.e. is it possible to install the same jar more than once or does it overwrite the exploded contents regardless of the initial jar name?