New FileInput widget

I had previously created pull request https://github.com/fdietz/team_dashboard/pull/83, however, I think this is a better solution.

Notes:
There are some files that I've dropped into the "vendor" directory, not sure this is the proper spot for these files.
I've added "overflow: hidden" to app/assets/stylesheets/application.css.less in order to get the behavior I need; curious if that is undesired.

Hope you dig it, it's working for us and solves a need we have!