Allow multiple default options/values when is_collection == True

[Zack-83]

Rationale / Begründung

Minimal functionality

For some questions accepting multiple answers (collections), multiple default answers should be acceptable, too.

Affected

Users, Managers

Additional features

Another possibility would be to assign different values depending on the answer of another previuos question.

Example: "Will the data be accessible through a free and standardised access protocol?"
--> If repository == "Zenodo" the answer is "via the REST API"
If repository == "PTB-OAR" the answer is "via the OAI-PMH protocol"