Toggling a task list item in a description with a mention creates a Todo for that mention

1. Create an issue with a mention and a task list:

   Hello @root
   
   - [ ] Task 1
   - [ ] Task 2
   - [ ] Task 3

2. As a second user, toggle any of the task items.

3. A "Mentioned by" Todo will be created for @root from the second user.

cc @dbalexandre