Fixes #6041: Members list pages (tabs/content/titles) can now be extended via plugins
Also moved total member count to sidebar
Add PHPDocs to members functions
Make sure empty string return is interpreted as "handling" the list hook
Adds message to popular tab when empty
Major simplification
Remove unneeded check for empty $vars['page']
Fixed docs
More members docs
Docs for the hooks
Sites probably won't need this.
Fixed docs
Merge pull request #6042 from mrclay/6041

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