There were two problems here:

 1. The periodic interval for updating the timestamp was never started
    for new annotations because the initial call to updateTimestamp()
    was a no-op for annotations that had no timestamp yet.

 2. domainModel.updated was never updated after an annotation
    was saved to the server and hence the call to
    updateTimestamp() re-used the old timestamp.

(1) is fixed now by restarting the interval using the current
last-updated timestamp whenever the view model is updated.

(2) is fixed indirectly now by moving the logic to update
the timestamp into updateViewModel(), so that all updates
to it happen in one place. When an annotation is updated,
the updated domain model returned by the server is the one
passed into updateViewModel() to update it.

 * Separate out the logic for creating a decaying interval
   whose frequency depends on the age of some input timestamp,
   and updating the timestamp string on each interval.

   The decaying interval is now handled by 'time.decayingInterval'

 * Trigger a restart of the interval whenever the view model is
   updated in response to a change to the annotation

Fixes #2819
Fixes #2822