Toggling a task in a description with mentions doesn't creates a Todo

What does this MR do?

When user toggle a task list item in a description with a mention it does not create an unnecessarily Todo for that mention.

Are there points in the code the reviewer needs to double check?

Nope.

Why was this MR needed?

To avoid creation of unnecessarily Todo for other users in the text.

What are the relevant issue numbers?

Closes #14116 (closed)

Screenshots (if relevant)

Not relevant.

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Documentation created/updated Not needed
• API support added Not needed
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together

/cc @JobV @stanhu