Several classes cannot be extended

[TickTackk]

Affected version

2.1.x

@Chris D Please allow \XF\Entity\BbCodeMediaSite::match_callback_class, \XF\Entity\HelpPage::$callback_class and maybe \XF\Entity\Page::$callback_class to be extended as well.

• BB Code Callbacks
• BB Code Media Site Callbacks

would be nice as well

(Basically every kind of callback unless there is a compelling reason not to allow extension)