AA-393: Update Next Section Logic

Inside content_highlights.py, we had code to calculate due dates
for when there isn't graded content, but we could only reach that
code path if the user had an assignment with a due date at the
target date. Now we will check for all learners who could be in
range of having an update and let the code in content_highlights.py
decide if a highlight should be sent

scripts/thresholds.sh

@@ -2,6 +2,6 @@
 set -e
 
 export LOWER_PYLINT_THRESHOLD=1000
-export UPPER_PYLINT_THRESHOLD=2430
+export UPPER_PYLINT_THRESHOLD=2400
 export ESLINT_THRESHOLD=5300
 export STYLELINT_THRESHOLD=880