Group "Members" page enhancements

Here are a few enhancements to the group "Members" page that I think would be quite helpful:

• Allow sorting: name, username, newest group member, oldest group member, most active, least active
• Show user bio snippet next to name
  • This can make it much easier to find who needs to be CC'd on an issue or MR (if the users actually put what they're in charge of in the bio...)
• Show member count on the top nav bar (like Issues, Merge Requests)