Fix scrollbar being cut off at end of gutter

Summary:
The "mlb" on the left nav creates a phantom bottom margin which gives the content measurable height but not scrollable height. Replace it with "plb" (padding) instead.

The 2px-spacer calculation was also not quite correct.

Test Plan:

• Viewed pages with navs; padding vs margin didn't seem to make any other differences.
• Scrollbar now stops in the right place in Safari, Chrome, Firefox.

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: epriestley

Differential Revision: https://secure.phabricator.com/D12007