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. 
We could potentially have another layer that stands in between the Manager and the file system. For what it's doing now, I thought it was not necessary to separate the concern since the code is not so complicated yet - it would when we started to add more validation logic, for example. But I can refactor it for this PR, if we want. Either way