Brand new annotations (and page notes) are currently displayed as editor
widgets in the sidebar alongside all the existing annotation cards. This
means they need to be sortable.

Sorting by location isn't a problem, because the annotation already
carries the document location information by the time it gets to
`addAnnotations`, but sorting by the updated date is, because currently
the `created` and `updated` fields are only provided when the server
sends back the saved annotation.

This commit changes that by adding temporary `created` and `updated`
timestamps to new annotations (those without an ID). These will be
ignored entirely by the server, and overwritten when the annotation is
saved.

If for whatever reason `addAnnotations` is called with a new annotation
that already has these fields, they will not be overwritten.

Fixes (the rest of) #96.