Work Items: Add edit button to Iteration widget

Adds distinct read/edit states to the iteration widget.

UX Details

• In read state, list iteration dates linked to the iteration detail and if present, the iteration or cadence name.
  • We will want to change this once iteration naming is in place, however for now we should follow the existing pattern.
  • This is slightly inconsistent from issues, where we show an iteration icon and make the cadence name the most prominent detail. As iteration exists only on issues and tasks, this is acceptable; no inconsistency is added (tasks already work this way) and the outcome is more legible. This could be a fast follow on legacy issues.
• When "Edit" is pressed, replace iteration label with a dropdown combobox; the combobox (specifically the search) should be focused automatically. Change "Edit" label to "Apply".
• Selecting an item via click or Enter applies the selection and closes the dropdown and returns to read state with the selected value shown.
• Clicking outside of the dropdown, pressing Escape, or clicking "Apply", closes the dropdown and returns to read state with the existing value shown.
• A loading spinner is shown with the label during transition from Edit to Read states.