Work items: Add edit button to labels widget

Adds distinct read/edit states to the labels widget.

UX Details

• In read state, list current labels as labels with clear button.
• Clicking a label opens the issue or epic list (depending on work item type) filtered by the label.
• Clicking the clear button removes the label immediately and does not initiate edit mode.
• When "Edit" is pressed, the list of labels is replaced by the label input.
• Clicking outside of the input, pressing Escape, or clicking "Apply", returns to read state with selected labels shown.
• A loading spinner is shown with the label during transition from Edit to Read states.

Labels input: This issue focuses on implementing the Edit button; the input is shown in designs as a dropdown which can be tackled separately.