plugins: Custom configuration

Allow plugins to define their own custom configuration pages. This is
provided so that plugins can define more sophisticated configurations above
the standard single-form configuration previously provided.
Fix help tip display if not loaded via AJAX
Merge pull request #1156 from greezybacon/feature/plugin-custom-config

Custom configurations for plugins

Sorta-Reviewed-By: Peter Rotich <peter@osticket.com>