Add two new template events that allow extension developers to add UI elements to the group management page within the user control panel.

This page in the ACP contains a few template events that can be used to add new form elements but the UCP version doesn't have any of these which means you can't replicate the ACP form in the UCP.

I propose the following events to be added within the GROUP WIDE SETTINGS section of this page:

• ucp_group_settings_before to add settings at the top of this form section.
• ucp_group_settings_after to add settings at the bottom of this form section.

I'll send a PR for this shortly.