fix issues pointed out in !6527

add task completion status feature to CHANGELOG

REF: https://gitlab.com/gitlab-org/gitlab-ce/issues/2296#note_2724697

Everytime the User check or uncheck a Task Item from the Issue or
Merge Request description, a new update is going to be
added to the activity logs of the Issue or Merge Request.

Note that when using the edit form, you can only update the Task item
status or add/delete/modify existing ones. Doing both actions is not
fully supported.

When nesting task list items, the parent item is wrapped in a `<p>` tag.
Update the task list parser to handle these paragraph wrappers.

Make the Markdown parser recognize "[x]" or "[ ]" at the beginning of a
list item and turn it into a checkbox input.  Users who can modify the
issue or MR can toggle the checkboxes directly or edit the Markdown to
manage the tasks.  Task status is also displayed in the MR and issue
lists.