fixed rendering and comment issues

The rendering issue where angular brackets would mess up the HTML has
been fixed. Also, it is now possible to write multi-line comments again.

frontend/src/components/submission_notes/base/FeedbackComment.vue

@@ -44,9 +44,7 @@
           <span>Won't be visible to student</span>
         </v-tooltip>
       </div>
-      <div class="message">
-        {{ text }}
-      </div>
+      <pre><div class="message">{{ text }}</div></pre>
       <v-btn
         v-if="deletable"
         flat

frontend/src/store/modules/submission-notes.ts

@@ -75,6 +75,8 @@ const submissionGetter = mb.read(function submission(state, getters) {
   let highlighted = state.submission.text || ''
   if (language !== SubmissionType.ProgrammingLanguageEnum.Markdown) {
     highlighted = hljs.highlight(language, highlighted, true).value
+  } else if (language === SubmissionType.ProgrammingLanguageEnum.Markdown) {
+    highlighted = hljs.highlight('plaintext', highlighted, true).value
   }
   // const highlighted = state.submission.text || ''
   const postProcessed = syntaxPostProcess(highlighted)