Fix #741: Narrow .portfolio_entry > .submodule

Before this fix, the interior of the div.module-body.contains-submodules
is 920px wide, but, including margins and borders, the .submodule is
888px wide, and the ul.action is 89px wide which is why the ul doesn't
properly float-left alongside the preceding clearfix div. Narrowing the
submodule div mitigates the issue on sufficiently wide viewports.

See https://openhatch.org/bugs/issue741 for test-setup instructions + before-and-after screenshots.