Add three new events to the `acp_groups.html` template file to allow extension developers to add more settings to the group management page within the ACP.

These new events would let extension developers add settings to the interface where admins can add members to groups.

Currently, this form has no event present which prevents extension authors from doing much (if anything) to these UIs.

I propose three events;

• acp_groups_add_user_form_before for adding settings at the top of the form.
• acp_groups_add_user_form_during which would let developers add form settings at the end of the standard list of options (make group leader, make group default) but before the textarea element where you enter usernames.
• acp_groups_add_user_form_after for adding settings at the bottom of the form.

I think it's very important to add three events, as they would all contribute to different use cases depending on what you were trying to achieve and importantly, help give developers more freedom in where their settings are placed depending on the hierarchy of the setting they want to add.

In particular, I want to add settings below the group leader and default group settings because it's less important but above the textarea where the usernames go because that should be the final interface element before you submit the form.