Tell the user when their trashbin elements will be deleted

[ChristophWurst]

As a leftover of #3118 there is no info for the users about when elements of the trash bin get deleted

Steps to reproduce

1. Delete an event in the currently viewed time range
2. Open the trash bin

Expected behavior

See explanatory text that tells me when elements in the trash bin will be deleted permanently. Wording could be "Elements in the trash bin are deleted after 30 days". Ideally we fetch the exact retention time from the server, so this might need a tiny change there.

Actual behavior

Users don't know when the elements vanish. They might think they stay there forever.

Calendar app

Calendar app version: Master after #3118