This introduces a new RB.ContentViewport class and

RB.contentViewport global that's used for tracking offsets into the

main browwser viewport that are safe for content. Its primarily useful

for helping dock content and align elements.
Elements can be registered for tracking, ensuring that any element

resizes automatically update the content area. The machinery for

tracking is only enabled when first used, to save on memory in places

where nothing is docked.
All tracking is done with a WeakMap. If the element disappears, it

will be untracked automatically. Worth noting, though, that the viewport

won't automatically adjust until another resize event occurs. We could

track removals down the road, but currently, it's not necessary.
The Unified Banner is registered as a tracked item on instantiation

during construction.
This will be used in an upcoming change to help properly center the

collapse button for diffs.

Unit tests pass.
Tested with upcoming changes for the diff collapse button, and

verified that any resizes to the Unified Banner were factored in

for positioning.