Task notes now disappear when they are deleted from the main task. Fix #11152 (#11167)

* Task notes now disappear when they are deleted from the main task.

* Html component changed back to Markdown. Markdown logic now accounts for if the value is an empty string.

* if-else statement to be sure that the markdown library doesn't create issues with empty strings.

website/client/directives/markdown.js

@@ -5,6 +5,9 @@ export default function markdown (el, {value, oldValue}) {
 
   if (value) {
     el.innerHTML = habiticaMarkdown.render(String(value));
+  } else {
+    el.innerHTML = '';
   }
+
   el.classList.add('markdown');
 }