[ticket/12329] Add <div> container to index blocks (online-list, etc.)

Adding a container to the special index blocks gives us much better control over layout/styling. Currently this is very hard (since there are no suitable CSS selectors).

I decided to use the `index-block` class for index_body.html and `misc-block` for the others. I did this because permissions and stuff like that is categorized under "miscellaneous" in the proSilver CSS files.

PHPBB3-12329