this commit is an attempt to make conv items better scalable on smaller screens. also save some horizontal space.

There is one minor issue: big images (without width given) posted to the wall are scaled down to 95% via max-width. it seems though that if there is no actual width given the scaling behaves different (minimum width is bigger at least in firefox) than if we have width at 95% and max-width at the actual image width.
In general conv_item.tpl and it's css are quite messy... they need a major revision... i'll see what i can do...
Extra wall-to-wall images are disabled for now.