Resolve "Sort staff on role and then username"

What does this MR do?

Sorts the list of staff returned on the overview page, first by Role (admin -> staff) then by name.

Screenshots

before

after

Does this MR meet the acceptance criteria?

• I have added tests to validate the new functionality.
• I have updated the documentation accordingly.
• I have added translations for all new client-side text. Closes #226 (closed)