Review Board has a lot of bits of UI that dynamically grow or shrink as

the page is loading. Diff fragments, for instance, dynamically load

after the page has otherwise loaded, which results in a page jump

(sometimes a significant one) when browsing through the page or after

having navigated to the anchor for a comment or review. There are also

elements further up the page that react as changes are made further down

(like the issue summary table when resolving/reopening an issue).
Some of this has been worked around in the past (the issue summary table

specifically), but the problem was pretty apparent in some browsers when

working with anchors on the review request page.
This change introduces RB.ScrollManagerView (accessible via

RB.scrollManager), which aims to fix the jumpiness as parts of the UI

update. When a view is replacing/showing/hiding a bit of UI, it can now

call RB.scrollManager.markForUpdate, passing in the element. After the

update, it can call RB.scrollManager.markUpdated. This tells the

scroll manager to schedule a scroll position fix, taking into account

the changes made to that element (and any others updated before the

scheduled position fix takes place). The scroll position will be

adjusted to compensate for the new heights/offsets in the updated

elements, keeping the current view of the page stable.
It also handles provides functions for navigating to an element or

position on the page, wrapping window.scrollTo(). This will take into

account an offset that views can adjust. This is intended for things

like the review draft banner, which would normally overlap the top of

the content being navigated to. The review request page has been updated

to use this whenever navigating to anchors.
Over time, this will be extended to offer additional functionality

related to the scrollbar and positioning on the page.

Tested this extensively on the review request page in multiple browsers.

Verified that anchor navigation worked consistently and took the floating

review draft banner into consideration.
Tested navigating to a page with an anchor and having the diffs load.

Verified that the page did not jump.
Tested that changing issue statuses didn't result in page jumps.
Unit tests pass.