Allow regular users to add a reason for editing their posts

Currently a regular user is not allowed to enter a reason for editting.
This is enforced by posting.php as only m_edit permissions are being checked.

This makes the feature not available to non-moderator users in any case.

'S_EDIT_REASON' => ($mode == 'edit' && $auth->acl_get('m_edit', $forum_id)) ? true : false,

https://github.com/phpbb/phpbb/blob/master/phpBB/posting.php#L1796

Instead why not add the permission level u_edit_reason to allow users to enter a reason for changing their post?