Fix bug: duplicated matters had wrong event date

For the not-so-often case where a matter was being
discussed at multiple events, the html ID was being set
based on the matter ID (which is the same across events)
instead of the event item id (which will be different for each event).
Thus, for a matter being discussed at multiple events, the code
was setting the event on multiple event items (that matched the matter id)
rather than just the one we wanted (for the event item).

By using event item id, we fix this. To verify this, look up items 
in district 5, and note that the “Sign Ordinance” appears twice, each
time with a different event date/time.

Fixes #48