Update URL handling to route even with missing unique ID

[aaronpk]

Since it's rare that an event slug will collide with another during the same month, it should be possible to link to events using an even shorter URL than the one that includes the unique ID at the end.

For an event like /2020/01/a-b-c-uniqueID, the URL /2020/01/a should redirect to the canonical URL. If there are more than one event possibilities for the given slug prefix then show all matching events. This ensures previously redirecting URLs will still show the correct event if a conflicting URL is added later.