Link forms & surveys answers to Custom User Property

The goal is that some of the answers given to a form and survey question can be saved as custom user property. This allows for some really powerfull use-cases with our audience selector + a easy way to enrich a user’s profile. Especially now that a Form can be part of the onboarding journey.

This needs the following changes:

1. Custom user property needs to support multiple selection for the Dropdown type

2. In the Custom user property overview we want to know if this field is populated from a Survey or Form

3. In forms we support the question types:

   1. Dropdown

   2. Slider (turns into dropdown)

   3. Yes/No

4. In surveys we support the question types:

   1. Multiple choice (turns in dropdown)

   2. Slider (turns into dropdown)