ical-publisher: Add description and link to event page, allow custom ical title

[twatzl]

Right now the events in the ical published calendar are rather useless.

They have neither a description nor a link to where to find more information.

Add an additional field publisher.ical.displayname for setting a custom name for ical publisher.
Display a link to the original source and add the possiblity to add an link to the published page. maybe as publisher.ical.moreInfoUrl.