Part of the Post-Interview Survey Feature

I'm considering this a bit of a spike to get the multi-select interface working. This just strikes me as the most difficult/highest risk portion of the feature, and it'll be good to explore it and knock it out asap. Things I think are worth kicking down the road to figure out separately in future issues:

• figuring out what the exact list of preexisting interview concepts will be
• styling
• copy that informs users about the survey and what the different fields are
• creating any way to view submitted surveys
• figuring out where the survey should be linked--we'll stick to visiting the path manually right now

Possible solution for the multi-select: stimulus wrapper around slim-select.

Acceptance Criteria / QA

1. Sign in as a normal user
2. Visit /interview_surveys/new path to reach a form to fill in for an interview survey. This is behind a feature flag, so this won't be available to users in production yet.
3. Fill in interview date for the survey
4. Use a multi-select field for interview concepts. This interface should allow the user to both:
   • Select any preexisting interview topics.
   • If a concept does not already exist, users are able to add a new concept.
5. Submit the form and create the survey. There won't be a UI wired up to view surveys, so will have to use the Rails console to check this.