- add order-by-clause to the corresponding JPA query (ordering by username ASC)
- adjust admin-client RoleResource to return a List instead of a Set, by introducing new methods #getUserMembers (instead of #getRoleUserMembers - the "Role" prefix is not needed, because it is clear from the resource name that it's about roles)
- adjust tests to use the new method and check that the expected order is returned
Closes#14772