[ticket/12535] Make <a> adjust to the image width

No HTML changes made.

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.

PHPBB3-12535