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

src/applications/differential/controller/DifferentialChangesetViewController.php

@@ -250,18 +250,20 @@ final class DifferentialChangesetViewController extends DifferentialController {
     $detail->appendChild($output);
     $detail->setVsChangesetID($left_source);
 
-    $output =
-      id(new DifferentialPrimaryPaneView())
-        ->setLineWidthFromChangesets(array($changeset))
-        ->appendChild(
-          '<div class="differential-review-stage" '.
-            'id="differential-review-stage">'.
-            $detail->render().
-          '</div>');
+    $panel = id(new DifferentialPrimaryPaneView())
+      ->setLineWidthFromChangesets(array($changeset));
+
+    $panel->appendChild(phutil_render_tag('div',
+      array(
+        'class' => 'differential-review-stage',
+        'id'    => 'differential-review-stage',
+        'style' => "max-width: {$panel->calculateSideBySideWidth()}px;"
+      ), $detail->render())
+    );
 
     return $this->buildStandardPageResponse(
       array(
-        $output
+        $panel
       ),
       array(
         'title' => 'Changeset View',