Code blocks should be horizontally scrollable

[stevengharris]

Now that code blocks are supported as a paragraph style, it's easy to end up with a code block that exceeds the width of the web view. This causes the entire page to be horizontally scrollable, which is annoying when the text properly wraps everywhere else. We can easily make the code block clip using css, but sometimes you want to see what's off the page. The way this worked before was that selected code blocks could scroll horizontally, and this needs to be put back in place with the new code block paragraph style.

[stevengharris]

This should be fixed by making the editor overflow-x: clip and code block overflow-x: scroll.