Read only questions in interview

[jochenklar]

Rationale / Begründung

It might be interesting to have questions in the interview, where users cannot change the value. In particular if a post_save Signal populates the values.

Affected

Users, Managers, Admins

Minimal functionality

Questions can be tagged as read-only (with a checkbox like optional). Input fields in the interview are disabled then. The API does not need to validate this.