Adjusts column width percentages for syllabus content

Changes the widths to allow proper resizing in Chrome, etc.
Removes the redundant 'Day' column. Changes the first column to
align left rather than center.

fixes CNVS-13924

test plan:
   - Go to a course syllabus in Chrome
   - Make sure several assignments are there with due dates
   - Resize the window to be small
   - Date and Day columns should not overlap but remain in their
     own columns.

Change-Id: I74a190d35ac9acacd1b179d520e8328b84ec34d5
Reviewed-on: https://gerrit.instructure.com/38802
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Hilary Scharton <hilary@instructure.com>
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>