Allow users to sort annotations by most-/least-recently-updated

We [landed a recent change](https://github.com/hypothesis/client/pull/3189) to make chronological sorting less "jumpy" to work with. In the referenced change, we made chronological sorting of annotations (which is the default sorting criterion for the Notebook view) use annotation `created` dates instead of the former use of `updated` dates. That is, before those changes, the "Oldest" and "Newest" options in the sidebar's sort menu sorted by when the (root-level) annotations were last updated; now they do what they sound like they'd do: they sort by root annotations' `created` dates.

Above: The sort menu in the sidebar. "Location" is the default sorting criterion.

It may be useful, in the future, to add sorting options for most- and least-recently-updated.