Work item note activity buttons enabled while loading

What does this MR do and why?

Fix some console warnings from missing labels on modals, and disabled buttons (related: Dropdown/Listbox warning text if button is disa... (gitlab-ui#3044 - moved))

User-facing change is the activity buttons on work item notes are now enabled during loading.

Changelog: changed

How to set up and validate locally

1. Load work item page (or any group page since that loads create-work-item-modal)
2. Check console for warnings about missing things
3. Rejoice
4. Load work item with notes (preferably lots)
5. Change activity filter/sort order while things are loading
6. Check that nothing is broken