Move markdown editor icons into app/frontend icons

These SVGs are used for the buttons in the markdown editor. The button
toolbar is only shown as a JS enhancement, so it makes sense that these
icons should be in the app/frontend/icons folder.

app/components/markdown_editor_component/_index.scss

@@ -63,23 +63,23 @@
   margin: 0;
 
   &--h2 {
-    background-image: url("../../assets/icons/markdown-editor-h2.svg");
+    background-image: url("~/icons/markdown-editor-h2.svg");
   }
 
   &--h3 {
-    background-image: url("../../assets/icons/markdown-editor-h3.svg");
+    background-image: url("~/icons/markdown-editor-h3.svg");
   }
 
   &--bullet-list {
-    background-image: url("../../assets/icons/markdown-editor-bullet-list.svg");
+    background-image: url("~/icons/markdown-editor-bullet-list.svg");
   }
 
   &--numbered-list {
-    background-image: url("../../assets/icons/markdown-editor-numbered-list.svg");
+    background-image: url("~/icons/markdown-editor-numbered-list.svg");
   }
 
   &--link {
-    background-image: url("../../assets/icons/markdown-editor-link.svg");
+    background-image: url("~/icons/markdown-editor-link.svg");
   }
 }