Public shares inherit default expiry date from file shares

[mejo-]

Describe the bug
When a default expiry date is set for file shares, this will apply to collective shares as well. There's no way to circumvent, extend or disable this in the Collectives app.

To Reproduce
Steps to reproduce the behavior:

1. Set a default expiry date for file shares
2. Create a collective share
3. Have a look into the file share details at "Collectives/<shared_collective" and see the expiry date
4. Wait until the expiry and see that the collective share is gone

Expected behavior
We should either enforce file shares without expiry or make the share expiry configurable in Collectives.