Locked questionnaires failing in editor form

[janno42]

#1445 introduced locked questionnaires. However, they are not dealt with correctly in the evaluation editor form. When initially opening the form, the locked questionnaires are correctly selected but are not handled correctly when saving the form.

Steps to reproduce:

1. As manager, assign a locked questionnaire as the only general questionnaire for an evaluation.
2. Enable the evaluation for editor review.
3. As editor, open the evaluation form and try to save it. Saving will fail with an error for the field "General questionnaires" ("This field is required.").

The locked questionnaire should count as a selected questionnaire and the form should be saved.
A test should be added for this use case.