[bugfix] corrects wide post content layout

Post content width and max-width has been set to 500px.
I didn't specifically did that before while I was designing my
template, because I already set parent width and sidebar width,
on the hope that parent_width - sidebar_width was a good formula.

It's not. Of course, for text-only posts, browsers automatically
set post content width to 500px. However, if wider content appears
(such as IMG elements or YouTube videos), the column expands,
making the layout mess up.

It's required to actually set the column width. It's not that
I didn't know that. I actually knew it, however I forgot to do that.
This bug was accidentally discovered by just live-testing the
template at my own website.