Migration to Nested Pages support

On upgrades from a pre-nested pages version to a version that only supports nested pages (most likely 2.6, to be decided on) a migration should happen that moves all events as nested pages of their corresponding calendar pages.

Things to take into account:

• the default calendar might be moved by the upgrade from MoccaCalendar.Events to MoccaCalendar.Events.WebHome - or it might be not if the users have customized it. Where to move the corresponding events?
• in the pre-nested pages version it is possible to have events in a different space than the calendar (e.g. by moving them into a different space after creation). If the space has different access restrictions this might have been used to intentionally hide some events. This needs to be handled somehow.