Comments: '@ mention' dropdown inaccessible for screen reader users.

Issue Summary

When a user wants to mention a person, by typing @, the drop-down with potential users is not read by the screen reader. First reported by FVillena here --> https://community.atlassian.com/t5/Jira-questions/Jira-accesibility-Screen-reader-when-mention-someone-with/qaq-p/1819658 

Steps to Reproduce

1. Turn on a screen reader
2. Go to an issue
3. Navigate to the comment section
4. Select "Add comment"
5. Type @ and try to browse to mention somebody
6. AT doesn't read the @ mention list, nor the selected user 

Screenshot

Screen recording

Screen Recording 2022-09-15 at 7.19.19 PM.mov

Actual Results

In the comment section, when the user tries to mention a user in the comment input field, a list of users is available. The list is accessible by the keyboard users, but the screen reader user is not able to access the listbox. This makes the screen reader user unaware of the listbox available and inaccessible of using the feature.

Expected Results

Screen readers should inform the user that a dropdown has been expanded, informing about the list of users, the length of the list, and the selected item.

• Provide role="listbox" to the <ul> element.
• Provide role="option" to the <a> elements.
• Use ARIA live region to announce the number of suggestions available.
  • <div role="status" aria-atomic="true" aria-live="assertive">5 results available</div>

Workaround

Currently, there is no known workaround for this behavior. A workaround will be added here when available