Add ical export

[rixx]

• Add ical export for single event
• Add ical export for all events
• Add ical export for all events of a speaker: add ical file for all talks of a speaker #304
• Add ical url for single event
• Add ical url for all events
• Add ical export for group of events
• Add ical url for group of events
• Add UI mode to define a group of events