This change adds an expansion icon to the right of the file index when

it's docked into the review banner. Clicking this will temporarily

expand the entire index, allowing the user to quickly switch between

files without having to scroll back up to the top of the page. There's a

subtle animation when this is opened, though closing happens

immediately.
This change also dramatically improves the behavior of scrolling when

anchors are clicked (filenames or chunk indicators in the file index) or

keyboard shortcuts are used to navigate. This is kind of tricky because

we have a lot of (literal) moving pieces--the page itself scrolls, and

the docked file index can change height. The height calculation inside

the file index has been exposed, and the page view runs the calculation

twice in order to dial in a good target scroll position.

Exercised the expansion icon a fair bit. Verified that the animation

  worked as expected and that the index was closed either when the icon

  was clicked again or when the page was scrolled (including clicking on

  an anchor to scroll to another part of the page).
Clicked on file index filenames and chunk indicators to scroll. Saw

  that the page always scrolled such that the target index was displayed

  a bit below the bottom of the banner, even when the banner was

  significantly changing size depending on the new scroll viewport.
Went through all the diffviewer keyboard shortcuts and saw that files

  and chunks were scrolled nicely.
Ran js-tests.