Resolve "Teachers submission view thinks latest submission is not latest submission when giving feedback."

• Problem was backend wasn't setting the isLatest field correctly for submissions (now fixed).
• Added the submission id on top of the feedback panel so it's easier for the user to identify which submission they are viewing.
• Made the textual feedback required since if you click 'save' without any feedback or grade it crashes.
• I saw we had some duplicate code which I removed.
• Made save button disappear if submission is not the latest (you cannot give feedback on past submissions)

Closes #146 (closed)