Allow some simple bootstrap options for user content

[Robert Lyon]

For example the collapse / show of content via <a> or <button> link

We should whitelist some safe options in the HTMLPurifier config, eg:

    $def->addAttribute('a', 'data-toggle', 'Enum#collapse');
    $def->addAttribute('a', 'aria-expanded', 'Enum#true,false');

To allow users to have access to more markup options

This could be useful for adding accessibility attributes as well

[Kristina Hoeppner (kris-hoeppner)]

We should check if there are others that should be allowed as well, like rounding images. There is another bug to allow more CSS3 things.