Convert all classes to BEM

This is an issue only if Drupal core fails to achieve this in a sensible manner.

So, basically implement BEM class naming as much as possible. Some we can do now (stuff the theme is soley responsible for generating) the rest we'll have to wait and see where D8 core ends up.

I have revisit the Drupal core guidelines on these, last time I looked I am sure they (Drupal core guidelines) were treating the element like a modifier which seems a bit whacky to me, but I might have read wrong.