Stop using JX.Scrollbar for main page content

Summary:
Ref T8151. This is option (5). It needs a few adjustments but feels pretty good. Major issues are:

• Without a mouse, the scrollbars overlap by default, so we must move the column off the right margin.
• Scrolling sometimes "bleeds" between the chat vs the main frame in a way that's not as discrete as the old framed content, but feels generally reasonable to me.

If we pursue this, I'd plan to make these additional changes:

• Move the panel away from the right margin only if the page scrollbars are zero-width (i.e., in OSX trackpad mode).
• Fix the notch in the upper right corner when the chat is moved away from the right margin.
• Probably remove the body "overflow-y: scroll" on Conpherence and Workboards.
• Update the resizing code to deal with 300px vs 315px widths.
• We can probably clean up some JX.Scrollbar "main panel" code.

Here's the "bad" case, where I've visually separated the column to provide room for a scrollbar. This isn't ideal, but looks and feels OK to me:

{F398375}

Test Plan:

• Tried Firefox, Chrome, Safari, with and without a mouse.
• Tried normal Conpherence.

Reviewers: btrahan, chad

Reviewed By: btrahan

Subscribers: avivey, epriestley

Maniphest Tasks: T8151

Differential Revision: https://secure.phabricator.com/D12789