Generate system note after Task item has been updated on Issue or Merge Request.

Reference: #2296 

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.

See merge request !1848