Expand feedback for study-specific visualizations & communication

# TL;DR
Use the existing feedback field to support researcher-provided links & other kid-specific material to give to families.

# Narrative
As a researcher, I want to show families that participate in my studies a graph of their child' performance in the task.

# Acceptance Criteria

- [ ] Researchers can add a link (single per study, or manually per-response) to the feedback page they want shown to a family.
- [ ] Help text and usage instructions are revised to indicate how the feedback feature should be used
- [ ] Add a new view for researchers to manage feedback (vs. consent & private notes)
- [ ] Families receive email (per notification settings) at new feedback content.
- [ ] Response page for families is redesigned to show feedback prominently when available.

# Implementation Notes

Initial version will involve link to researcher-hosted content supporting e.g. child/response slugs, not iFrames/within-Lookit display of material.

See these issues for associated improvements to Feedback feature: #152, #150, #615

Proposed in a grant by Juliana Girard, to submit April 2022

Start by tracing the existing feedback field and its family response flow, then read related issues #152, #150, and #615. Use the acceptance criteria to map researcher management, help text, notifications, and the family response redesign. Done means researcher-provided links can be configured and shown to families with the specified email behavior.

Written by the indexing model from the issue text.