Rename `task` (markdown checklist item) to `check list` in various places throughout the UI

Problem

• We are making Tasks a first-class work item type.
• This name collides with markdown checklist items, which are also called tasks.

Proposal

Throughout the UI, rename references to markdown checklist tasks to checklist (singular checklist which contains many items) and checklist item (singular item within a checklist)

Locations:

• System notes
• Issue row within issue lists
• Issue header (currently reads x of x tasks completed)
• Editor (Add a task list)

Context	Current	New
System Notes	marked the task [name] as completed	marked the checklist item [name] as completed
Issue list	0 of 3 tasks completed (hidden at small size)	0 of 3 checklist items completed (hidden at small size)
Issue header	0 of 3 tasks completed 0 of 3 tasks (at small sizes)	0 of 3 checklist items completed 0 of 3 checklist items (at small sizes)
Editor	Add a task list	Add a checklist