Fix places where we were using relative imports.

Relative imports won't be allowed in Python 3, and they sometimes cause very
mysterious bugs (like decorators being applied twice).

Reviewed at https://reviews.reviewboard.org/r/4929/
Update octal literals to use the new syntax.

Python 2.6 introduced a new syntax for octal literals, 0oXXX, which is more
consistent with other non-decimal literals. This syntax is required in Python
3, so we might as well update everything now.
Shrink the width of the diff file index image in the release notes.

The image was just too wide for the website, causing it to jump below
the sidebar when the page was at its minimum width. It's now been
resized to reduce the amount of horizontal whitespace within the image,
to kept it below the required image width.