show loading indicator while waiting for assignees first fetch

What does this MR do?

Fixes:

• Assignees section tiny and empty when first loaded
• "Assign to me" shown before Assignee loaded

Are there points in the code the reviewer needs to double check?

• is there a preferred way to flag the initial "don't display yet" state? isFetching could be more accurately called isInitialized
• should Time Tracking be updated to do this as well?

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• [ ] Documentation created/updated
• [ ] API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #32170