Previously, the `AnnotationHeader` component checked the equivalency
of `annotation.created` and `annotation.updated` to determine if an
annotation had been updated subsequent to its creation. This is
problematic if an annotation has a very small difference between its
`created` and `updated` dates.

`created` and `updated` are returned by the API as ISO-8601 date strings
with microsecond resolution. These days, annotations created by the `h`
service will have exactly equivalent `created` and `updated` timestamps
(assuming they haven't actually been edited subsequently, of course). In
the past, however, annotations often had negligible (sub-second)
differences in their `created` and `updated` dates. This resulted in
an edited timestamp misleadingly appearing in the UI for older
annotations.

These changes add a `hasBeenEdited` function to the
`annotation-metadata` util module that considers an annotation edited
only if `created` and `updated` differ by at least 2 seconds.

Tests for `annotation-metadata` have also been modernized somewhat here.

Fixes #2684