Work items label widget input polish

What does this MR do and why?

• Add "clear all" button to labels selector when input is focused
• Don't reorder labels when updating from server. Keep current order, add any new ones to the end
• Clear "active" state if dropdown is closed without a selection

before	after
labels get reordered after adding (server alphabetizes). No "Clear all" button. Border stuck open if opening dropdown without making selection	Fixed
before	after

Testing

1. Enable work_items_mvc_2 feature flag
2. Create a Task on an issue (gray box below description, New task), click task title to open modal
3. Add some Labels
4. Use 'Clear all' button
5. Try opening labels dropdown, then click outside, check gray outline box disappears

Related to #377990 (closed)