Calendar subscriptions from imported files need to be editable

Currently, if a user or admin uploads a calendar according to the directions in https://docs.moodle.org/29/en/Calendar_import Moodle will treat it as a calendar subscription and prevent the imported events from being edited or deleted.

This approach makes sense when the subscribed calendar is via a link (i.e. linked to a Google calendar) because any edited events will be overwritten as soon as the calendar is refreshed (see https://tracker.moodle.org/browse/MDL-36276).

However, this does not make sense for calendars imported via a file since they are not linked to anything. Most often, these events are uploaded in a "one-shot" manner, for example, if a school wishes to populate the Moodle calendar with events for the entire year. However, in the aforementioned example, it is often quite necessary to make changes to these events and having to edit another document followed by uploading the calendar every time a change is needed makes for a terrible workflow.

Therefore, when dealing with calendar imports:

• events created via uploaded files should be allowed to be edited and deleted
• events created via calendar links should remain as is, not editable