Implement a way to add new deadchat questions

[mvarendorff]

Currently the only way to add more !deadchat questions is to insert them directly into the database. This should be possible in the same way as adding daily-challenge questions.

Blocked

This issue is currently blocked by the lack of support for interface types in Prisma

Implementation

To be determined because I don't know how this works with Prisma

[mvarendorff]

This is currently not (cleanly) possible due to prisma/prisma#2506

Given this feature isn't used often and adding deadchat questions manually isn't much of a problem, I would like to wait for a native Prisma solution for now.