Add php event for modifying the data when composing a PM

Identifer: core.ucp_pm_compose_alter_vars
Location: /includes/ucp/ucp_pm_compose.php (after [c]$user->add_lang('viewtopic');[/c])
Parameters: 'msg_id', 'to_user_id', 'to_group_id', 'submit', 'preview', 'action', 'delete', 'reply_to_all'
Explanation: Allow Extensions to modify the default vars before composing a PM. The current events are only called if $sql != '' (which would be false in the case of posting a new PM). Example code:

// A new permission that allows users to reply to PMs, but not compose new ones

public function ucp_pm_compose_alter_vars($event)

{

    $action = $event['action'];

    if ($action == 'post' && !$this->auth->acl_get('u_post_pm'))

    {

        throw new \phpbb\exception\http_exception(403, 'NOT_AUTHORISED');

    }

}

I can start on a pull request shortly if there are no objections or issues with the proposed event.

Area51 topic: http://area51.phpbb.com/phpBB/viewtopic.php?f=111&t=47256