When adding code blocks with certain bits of text (such as a leading

shebang, Python-Markdown) can go into "line number mode", where it tries

to render the code blocks with line numbers on the side. A shebang seems

to trigger this behavior. When in this mode, some assumptions we make

for Markdown diffing fail, leading to a crash.
This change forces line numbers to be off, so that we're never in this

state. Now, all code blocks behave exactly the same way.
While testing this, I noticed that our rendering of changed code blocks

for change description entries was incorrect. Code blocks were shifted

and text was obscured with a background color. At some point, we may

need to completely rethink the CSS around all of this, but for now, new

styles have been written to make these flush with the container, fade

out the background, and include top/bottom borders around the code

block (which used to exist a few major versions ago).

Tested with some code samples that formerly caused crashes.
Unit tests pass.
Diffed several Markdown code blocks and saw that the results were

legible.