External calendar is added multiple times

[mpfau]

Describe the bug
When I add a new external calendar subscription I might click on subscribe multiple times as the UI does not update after the first click.

To Reproduce
Steps to reproduce the behavior:

1. Add Calendar / from URL
2. Paste ics link
3. Click on subscribe
4. Click on subscribe
5. Click on subscribe
6. The subscription will be added multiple times

Expected behavior
The UI needs to change after I click on subscribe for the first time. The calendar should only get created once.