R1:e0b9a6338881

make Differential comment panel flexible width and nip a bit at other flexible width issues

Summary:
all sorts of stuff

- made comment form width flexible
- made margins element specific rather than part of differential-primary-pane
- made box elements all veritically align left and right until code stuff
- re-factored width calculaton stuff a bunch so only the code section has to suffer from max-width calculations; everything else can flex
- made colspan 3 for rightmost table header element. this…