Explore page improvements

The public groups directory (Explore) is easier to browse and easier for admins to control.

Sorting and listing

Explore now offers four sort orders for the groups directory: most members, fewest members, newest, and oldest. Group admins get a clearer "List in the groups directory?" choice in group settings, with better-worded descriptions of what listing means for privacy. "Anyone can join" is available as a group option even when public account creation is closed at the instance level.

Restricting Explore to signed-in users

Instance admins can now require sign-in to view the Explore page. When enabled, visitors who aren't signed in are sent to the login page instead of seeing the public groups directory.