[ticket/12535] Improve postprofile avatar block

A reworked version of https://github.com/phpbb/phpbb/pull/2451

>Changed so that the `<a>` that surrounds the `<img>` in viewtopic does not use
>all width and, instead, have the same size as the image.
>Additionally, I added a CSS pseudo-element to make the required forced
>line break after the `<img>` happen.
>
>Only limited testing made. Please test before merging.

https://tracker.phpbb.com/browse/PHPBB3-12535
https://area51.phpbb.com/phpBB/viewtopic.php?f=81&t=45609

PHPBB3-12535
The avatar in the header is not a floating block element that needs to be contained and cleared.

In the header, the avatar is an inline element. The container there is the `<li>`.