External calendar might get synchronized multiple times in parallel

[mpfau]

Describe the bug
If I try to synchronize an external calendar multiple times an actual sync is triggered multiple times in parallel. This leads to errors (e.g. BadRequest) due to race conditions.

To Reproduce
Steps to reproduce the behavior:

1. Add an external calendar
2. Click on sync
3. Click on sync
4. Click on sync

Expected behavior
Only one sync should be run at a time and while it is running it should not be possible to schedule another sync.