added a hint on commented lines when comments are hidden

frontend/src/components/submission_notes/SubmissionCorrection.vue

@@ -8,6 +8,7 @@
       <template slot="table-content" id='sub-lines'>
         <tr v-for="(code, lineNo) in submission" :key="`${submissionObj.pk}${lineNo}`" :id="`sub-line-${lineNo}`">
           <submission-line
+            :hint="hintAtHiddenComment(lineNo)"
             :code="code"
             :line-no="lineNo"
             @toggleEditor="toggleEditorOnLine(lineNo)"
@@ -170,6 +171,10 @@ export default {
         }
       }, 5e3)
     },
+    hintAtHiddenComment (lineNo) {
+      return !this.showFeedback && this.origFeedback[lineNo]
+        && this.origFeedback[lineNo].length > 0
+    },
     init () {
       SubmissionNotes.RESET_STATE()
       SubmissionNotes.SET_SUBMISSION(this.submissionObj)

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

 <template>
   <div>
     <td class="line-number-cell">
-      <v-btn
+      <v-btn v-if="hint"
         block
+        depressed
         class="line-number-btn"
+        color="error"
+        @click="toggleEditor"
+      >
+        {{ lineNo }}
+      </v-btn>
+      <v-btn
+        v-else
         flat
+        block
+        depressed
+        class="line-number-btn"
         @click="toggleEditor"
       >
         {{ lineNo }}
@@ -32,7 +43,11 @@ export default {
     codeLanguage: {
       type: String,
       default: 'lang-c'
-    }
+    },
+    hint: {
+      type: Boolean,
+      default: false,
+    },
   },
   methods: {
     toggleEditor () {
@@ -60,5 +75,6 @@ export default {
     height: fit-content;
     min-width: 50px;
     margin: 0;
+    border-radius: 0;
   }
 </style>