Allow adding submission comments for submissions without a grade

[JasonGrace2282]

Currently, we only allow adding comments to submissions with a grade:

tin/tin/templates/submissions/show.html

Lines 244 to 259 in 7a18b1c

	{% if submission.complete and submission.has_been_graded %}
	{% if is_teacher or request.user.is_superuser %}
	<h2 style="border-top:1px solid lightgray;padding-top:15px;">Add Comment</h2>
	<form action="{% url 'submissions:comment' submission.id %}" method="post">
	{% csrf_token %}
	<textarea name="comment" placeholder="Comment" rows="5" cols="120"></textarea>
	<br>
	<br>
	<label for="point_override">Point Override:</label>
	<input type="number" name="point_override" value="0" min="-999" max="999" step="0.5">
	<br>
	<br>
	<input type="submit" value="Add Comment (and publish)">
	</form>
	{% endif %}
	{% endif %}

Ideally we should allow adding comments to any submission, regardless of if it has a grade or not. However, there are a multitude of places within Tin's code where it assumes that the submission has a grade when dealing with comments.

Cue the Mission Impossible music: your mission, should you choose to accept it, is to allow adding comments to ungraded submissions, without introducing any new logic bugs. This message will self-destruct in 3. 2. 1.