Of course, this means that there's a bunch of logic
to coordinate the new flag with the original -
`automaticallyCollapsed` - flag.

In addition: the UI is updated to use the new flag
whenever it's been set, but to continue to fall back
to the old flag. This is primarily facilitated by a new
helper utility for diff files to do the resolution in a
single centralized place.

Of note: the diff_file is updated to depend on the
global store as the authoritative source of determining
whether the file is collapsed or not. The previous
version kept a local `isCollapsed` in sync with the
global store, which introduced a huge amount of
interwoven complexity. The update ingests the
resolved collapsed state from the store when
various observed properties update instead of
inverting that flow.

This is a full refactor of file collapsing.
Because the previous version was a single flag, then
modified with a user preference (file-by-file), there
were some issues including a THIRD possible way
that collapsing works. This update slightly simplifies
how collapsing works by depending more heavily
on the two flags stored in state and only modifying
them with the user preference when necessary.