R1:bf3dd8663c00

Add "buoyant" headers to Differential

Summary:
As you scroll through a diff, add a fixed-position header to the top of the
document to provide context. This is particularly useful with keyboard
navigation.

The technical implementation is that we seed the document with invisible
markers. When the user scrolls past one, we show a header with that text until
they scroll past another.

Test Plan:
Scrolled through a revision, was presented with context.

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