[course_admin/submissions] lighten CSV outputs

[anthonygego]

Is your feature request related to a problem? Please describe.
When you filter submissions in the submissions view and then download the CSV, you get all the submission info, feedback included. The later may be very large and not useful in a CSV download, especially if you only want to draw charts from the data.

Describe the solution you'd like
We should be able to select or filter out rows (that could be in a modal for instance, and the checked rows can be saved in cookie, session, or whatever), or to have a lighter download options.