iCal: Add indication on how long the import is going to take

[JoeyStk]

Motivation

Since #100 it's possible to import events from external calendars using the ical format. However, the import can take a while and currently there is no indication on how long it's going to take.

Proposed Solution

Therefore we want to make a few adjustments to give more feedback on what to expect.

• Don't import on the main thread
• Show a message on how long the import is probably going to take

Alternatives

None others were discussed

User Story

As a user of the CMS I want to get an overview on how long the import is approximately going to take and get some feedback so that I know what's going on and whether or not this is expected or an unexpected error which I should abort.

Additional Context

None

Design Requirements

None