Update annotation card timestamps regulary

For annotation timestamps we use the fuzzyTime filter
which subtracts the annotation.updated timestamp from the current time
to show the time passed after this annotation was modified.

In the past it was updated regulary however after merging the
view-sort overhaul branch it was removed and by default an annotation
scope didn't receive any digest cycle unless user interaction happens
on that card.

So the timestamps weren't updated regulary.
This fix does nothing more than triggers a digest cycle at every
15 seconds for every annotation scope. Because filters are recalculated
at every digest scope, the timestamps at the cards are updated too.

Since annotation directive uses isolated scope the digest cycle doesn't
blow up, and nothing outside of this scope is affected.

As for the two scope.destroy functions, both of the are executed.

Fix #1615
Originally the filter was: {{annotation.updated | fuzzyTime | date:mediumDate}} I wanted to preserve this[1] . 

Now the docs[2] says: `Formatting rules (see Description). If not specified, mediumDate is used.`
So that is why no format parameter is given.

[1] https://github.com/hypothesis/h/blob/master/h/templates/client/annotation.html#L40
[2] https://code.angularjs.org/1.2.26/docs/api/ng/filter/date