Improve UX when submitting feedback

Show a "feedback received" alert and hide the Submit button after
submission to signal students that their feedback has been received
and they can move on.

Fixes #122

Author: murhum1

feedback/locale/fi/LC_MESSAGES/django.po

@@ -842,6 +842,10 @@ msgstr "näytä lisää"
 msgid "This feedback has unsaved changes!"
 msgstr "Palautteessa on tallentamattomia muutoksia!"
 
+#: feedback/templates_j2/feedback/_form.html
+msgid "Feedback received."
+msgstr "Palaute vastaanotettu."
+
 #: feedback/templates_j2/feedback/_form.html
 msgid "Submit"
 msgstr "Lähetä"

feedback/templates_j2/feedback/_form.html

@@ -164,6 +164,10 @@
 			{{ _("This feedback has unsaved changes!") }}
 		</div>
 
+		<div id="feedback-received-message" class="alert alert-info mb-0" style="display: none">
+			{{ _("Feedback received.") }}
+		</div>
+
 		<div>
 			<div class="btn-toolbar gap-2">
 				<input
@@ -182,6 +186,13 @@
 	<script>
 		(function($) {
 			const f = $('#{{ form_id }}');
+			// implement feedback received notification
+			// needs to be only shown when feedback has just been sent by student
+			const feedbackJustReceived = f.closest('.exercise-response').data('aplus-grader-feedback-just-received');
+			if (feedbackJustReceived) {
+				$('#feedback-received-message').show();
+				f.find("input[type=submit]").hide();
+			}
 			// implement show more
 			const c = f.find('.history-well'), m = c.find('.show-more');
 			m.click(function() { c.find('li.initially-hidden').removeClass('initially-hidden').show(); m.hide(); });