Issue #1726

**Issue:** #1726

**Description:** CSV Uploading: Upload buttons should be disabled if no file is selected. This is for all CSV upload fields.

**Fix:** A javascript file was added to the assets folder called `upload_button_control.js`. This script waits until the HTML file upload control on a page changes and then will enable or disable the upload button (depending on whether or not a file was selected). Any upload form that requires for buttons to be disabled if no files are selected just needs to add the script to the page and pass the id of the upload control and of the buttons to the script.

This feature has been implemented on the following pages:
* Assignments Main Page
* Assignments -> Groups Page
* Assignments -> Graders Page
* Users -> Students Page
* Users -> TAs Page

**Testing:** Each page that was implemented with this feature was tested to ensure buttons were properly being enabled and disabled. Additionally, the rspec test was carried out with no failures or errors.
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