Rename Child widget to Tasks for MVC and update the Add button & help copy to reflect this

Context

Following up on conversation here: #369882 (comment 1057880373)

In our first iteration of Child items there are a few challenges:

• Only Tasks are displayed and addable within this widget
• There is no clear open/close Task icon or validated way to display this status separated from indictor of work item type (used to be an open/close icon)
• There is no clear way to indicate type of item within this widget and issue type icons are receiving feedback and may need to be iterated on.
  • To the above point, there are only tasks in this widget anyway meaning it should be clearly labeled as such.
  • We currently rely on the Add a task button to demonstrate type when clearer labeling would be the preferred solution.

Solution

1. Change the title of this widget from Child items to Tasks (for now) until we add other types to this widget and sort out some of the above challenges
2. Change that Add a task button labeling to just Add as it is now clear what the user is adding based on label alone.
3. Change help text copy from: No child items are currently assigned. Use child items to prioritize tasks that your team should complete in order to accomplish your goals! to No tasks are currently assigned. Use tasks to break down the goal of this issue into smaller parts.