Allow admins to check the public checkbox by default on specific group post types for a specific group type.

Issue #1123524 by Patrizio, KaleWorsley: Allow admins to restrict the available membership request options for a group type.

feature #769400 by miraclestyle: Added support for audience checkboxes and descriptions field. Also some code style tweaks and refactoring the UI.

feature #843382 by izmeez: Allow users with 'administer organic groups' permissions to bypass enforced defaults.

Adding 'og_is_group_type' to 'og_defaults_form_node_type_form_alter' to only show group settings on group nodes.