Feedback page should use all feedbacks for summary

Now that teachers cannot see feedback made outside their course anymore, the graph of scores only shows scores from managed courses as well. The graph should include all feedback scores (even those made outside of managed courses).

• Change feedback ordering to have the newest feedback at the top of the page
• Fetch the normal feedback for the summary

What else needs to change as part of this issue?

Some other things changed as part of MR undertaken for the issue, here is a comprehensive list:

• PageUtil was refactored to require a comparator as PageImpl does not take care of sorting (nor slicing)
• Refactored a bunch of FeedbackRepository methods to return lists instead of pages. Makes the code cleaner as we can just use PageUtil at the very end.
• The count was already considering all feedback, not just feedback under manager courses. The confusion stemmed from the fact that the graph would only be rendered if there was written feedback to accompany the rating. The graph will now always be rendered (see SS in image).