Grey-out and disable import button when no file selected.

Per [70900178](https://www.pivotaltracker.com/story/show/70900178).  Problem was that users a) could click on the upload button when it appears disabled, and b) it would appear disabled even in cases where it's valid to use, and c) they get an unhelpful message if clicked on without a file.

This resolves all of the above by enforcing the disabled appearance only when there is no file, and prevents the form submission altogether if the file is missing.