iCal: Add filter to event list to filter for imported events & not imported events

[JoeyStk]

Motivation

Since #100 it's possible to import events using the iCal format. However this can overcrowd the event list quite easily. Therefore we want to add filters to the event list.

Proposed Solution

The idea would be add two options to the existing filters:

• Make it possible to filter for imported events
• Make it possible to filter for not imported events.

Alternatives

No others were discussed

User Story

As a user of the cms I want to have an overview of which events have been imported/have not been imported so that I'm not too overwhelmed when working with events.

Additional Context

None

Design Requirements

Think about a sensible input field