If there's an anchor in the URL in Differential, don't stick to the bottom of the page as content loads

Summary:
Fixes T11784. A lot of things are interacting here, but this probably gets slightly better results slightly more often?

Basically:

• When we load content, we try to keep the viewport "stable" on the page, so the page doesn't jump around like crazy.
• If you're near the top or bottom of the page, we try to stick to the top (e.g., reading the summary) or bottom (e.g., writing a comment).
• But, if you followed an anchor to a comment that's close to the bottom of the page, we might stick to the bottom intead of staying with the anchor.

Kind of do a better job by not sticking to the bottom if you have an anchor. This will get things wrong if you follow an anchor, scroll down, start writing a comment, etc. But this whole thing is a pile of guesses anyway.

Test Plan:

• Followed an anchor, saw non-sticky stabilization.
• Loaded the page normally, scrolled to the bottom real fast, saw sticky stabilization.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11784

Differential Revision: https://secure.phabricator.com/D17911