Mybb: Hooks to allow plugins to extend custom moderation tools

[JordanMussi]

Plugins are currently not able to extend custom moderation tools to provide additional options because of a lack of useful hooks:

There are no hooks in inc/class_custommoderation.php whereas there are in inc/class_moderation.php.

The hooks that are in admin/modules/config/mod_tools.php aren't quite sufficient for adding extra options.

Although a very nasty solution is to use the admin_form_output_submit_wrapper hook and filter on the current modules loaded to add extra form fields but that doesn't resolve the need for hooks in the custommoderation class.

[euantorano]

Note: I've set this as a target for 1.8.24 rather than 1.8.23, so please don't merge any associated PRs until 1.8.23 is released.