Fixes #6041: Members list pages (tabs/content/titles) can now be extended via plugins

- [x] Add docs for new plugin hook
- [x] Devs can extend list pages/tabs via hooks
- [x] Page titles now can change with tab
- [x] Moves member count to sidebar (this was confusing in the title of pages like "online")
- [x] Adds message if popular tab is empty (no site friendships)