Allow selecting text in new files again.

The CSS that made text in unrelated columns unselectable when selecting text
wasn't correct in the case of new files (where the "number 1" column uses td.r
for styling). To fix this, I've added a :not selector to verify that we don't
unintentionally disable selection for new files.

Testing done:
Tested text selection in both new and changed files.

Fixes bug 3661.

Reviewed at https://reviews.reviewboard.org/r/6609/