Only show calendar pages of the current space when creating new events

As suggested by Serge Marcotte in a comment of the extension page

when the filter = "space" it will be nice to have only the calendar that are in relation of the space when I create a new event.

Use case:

• several spaces contain their own event / calendar structure
• e.g. space A has an "deadlines" calendar which shows their deadlines and space B ditto a "deadlines" calendar firth their deadlines
• when creating an event in space A, only the "deadlines" of space A should be shown, not the on from Space "B"

Currently UI support to separate calendars in spaces is quite poor; one have to create the calendar page in the main calendar app and move it to the corresponding page.

There is an alternative use case where the calendars are created as global "categories" ; i.e. there is only one "deadline"
Calendar, and both Space A and Space B use it to create their events (which after fixing MOCCACAL-75 end up in their proper spaces).
need some idea to distinguish between the two use cases