Simplify side nav CSS and fix various glitches

Summary:
Fixes glitches in the side nav. Resolves T1828. Resolves T2156.

• Elastic scrolling (T2156): in Safari on OSX, using a scroll touch on the trackpad to scroll up past the top of the document caused newer-style side menus to scroll down, leaving a visible whitespace bar.
• Whitespace glitch: Particularly in Safari, scrolling down the document quickly from the top caused the top menu to scroll away before the side menu rose to meet it. Use a fixed background color bar that extends under the menu so this doesn't happen.
• Use of "!important": use CSS better so we don't need to "!important" things.
• Dark Console (T1828): Instead of hard-coding the top position, determine it dynamically by looking at where the content is. This also fixes the menu overlapping with the red "there are errors on this page" development bar.
• General "fixed" glitchiness: don't use fixed-position for menu content other than flexible (draggable) menus.

Test Plan:

• Viewed and scrolled menus in Paste. Opened and closed DarkConsole. Switched devices.
• Viewed and scrolled flexible menus in Differential and Diffusion. Opened and closed DarkConsole. Switched devices.

Reviewers: vrana, chad, btrahan

Reviewed By: vrana

CC: aran

Maniphest Tasks: T1828, T2156

Differential Revision: https://secure.phabricator.com/D4185