Comparisons are broken for Panel Surveys

[matiasgarciaisaia]

When creating a new Panel Survey, if you enable either of both comparisons (questionnaire comparison, or modes comparison), an incomplete Comparison step appears in the sidebar - but the section in the form doesn't.

This prevents the Start button to appear (the survey remains incomplete), and clicking the Comparisons button on the sidebar throws an error in the UI.

I'm not sure if we should disable comparisons for Panel Surveys or if we have to make them properly work.

Seen as of 97d25c7