Resolve "Mark commented lines when feedback is hidden"

Closes #160 (closed)

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>