assume clubs exist when adding callink events

when associating callink events with their club we assume that the event
exists

callink_events feature has been changed accordingly to reflect this
assumption

TODO: make sure that the rake task that import callink events and groups
imports the groups first
parse and save callink permalink in db
save facebook_id when importing facebook_events
methods to query facebook/callink events

to query: use Event.facebook or Event.callink

also added inflection methods so that an event can tell whether or not
it is a facebook or callink event

additionaly all callink events have a non-null callink_permalink
property
convenience methods to get events by time + range

includes methods to get events by day,  by datetime, or date range and
get events within the next week
clean facebook_url before Club validation

- Callink's organization api does not always have valid urls, which
motivates us to clean the url before validation. In our case, one of the
organization's facebok urls were duplicated so we extract the first valid url.

- validate facebook_url if facebook_url is present