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. 
Yeah, we definitely need to more validations. I'd agree with another PR that handles all possible validations - valid jar, duplications, enough disk space, etc. We probably want to brainstorm how we want to handle and what error message we want to return to the user in each case - some kind of AC would be helpful for the PR.