R1:404fe482d914

Add a Quicksand-aware page-level container to diffs

Summary:
Ref T12616. Ref T8047. Ref T11093. We currently have several bugs where diff state sticks across Quicksand pages.

Add a new top-level object to handle this, with `sleep()` and `wake()` methods. In `sleep()`, future changes will remove/deacivate all the reticles/editors/etc.

See T12616 for high-level discussion of plans here.

This general idea is likely to become more formal eventually (e.g. for "sheets" or whatever we call them, in T10469)…