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 is so the "View Options" UI element ends up lining up correctly with the "Show All Lines" element just below

Test Plan: looked at revision view and changeset view and it all looked hot. note I did not test what things looked like with different word wrap values; that should still work given the re-factoring and not re-design here. also toggled haunted panel mode and it looked good.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T2006

Differential Revision: https://secure.phabricator.com/D3866