Reporting: do not duplicate isstale functionality

The interaction entries have a isstale() method, we do not need a
template tag, that tries to figure out this on its own. The isstale()
method works even better: It knows, if the interaction is the current
interaction and compares the timestamp to the current time or (if it is
not the current one) it compares the timestamp to the timestamp of the
next interaction. So using the method of the model, you can browse the
interaction history and see, if the host was stale some time in the
past. Previously all interactions more than 24h ago were marked stale.