SCOPING: Allow adults/parents to participate

# TL;DR

Researchers sometimes want to just collect data from parents, and we don't have a clear way to do this. When researchers do run studies meant just for a parent, it is confusing for families.

# Description

Right now, there's no easy/clear way for researchers to run a study that is just meant for the adult/caregiver. The main barrier to this is that our recruitment emails and participation pages assume that the study is meant for the child.

1. On the participation page, there is no separate layout when the user is the intended participant (e.g. remove the child dropdown selection or add a "self" option).
2. We don't have a way to send out recruitment emails that are directed to the adult (i.e. "You are invited..." instead of "Your child is invited..."
3. In our database, the study responses are associated with a _child_ account, and it's not possible for this field to be null or for a user ID to be used instead.
4. Our experimenter pages for handling consent, responses and messaging assume that there is a child account associated with the response.
5. The filter categories on our family-facing study list page are all child-related - there are none for adult participation.

There are a few other things that are not ideally set up for adult participation, though these issues may not be a barrier to introducing this feature. For instance:

1. Most of the study exclusion/inclusion criteria (age, languages, etc.) relates to children, not to adults.
2. We don't collect much demographic data on the adults, and this is something that researchers may want/need in order to determine eligibility.

# Scoping

- Alter the **response model**/table and/or **child model**/table to allow the user to be the participant.
- Update the **study model**/table and **study create/edit form** to allow researchers to indicate that their study is only meant for adults.
- Update any **experimenter pages**/views that assume the participant must be a child
- Add a filter to the **family-facing study list** for adult-only studies.
- Add/edit the **study announcement email template** for adult-only studies.
- Add tests

This MVP would lack adult-focused eligibility criteria and additional demographics questions. There would also be some confusing language on the experimenter side, e.g "child" label in response downloads.

## Implementation

**Option 1**

We could alter the response model/table to allow the child ID column to be null, and add a boolean column for 'adult participant'. This way, we would no that it is a response to an adult-only study and that the user ID should be used as the participant, rather than a child ID. We would need to update any pages/views that access child-related info (ID, name, demographics) and change these to check whether 'adult participant' is true, and if so use the user ID/info, otherwise continue to use the child ID/info.

**Option 2**

We could automatically create a 'child' account based on the user. Currently, each user account has 0+ child accounts associated with it. With this option, a user account would automatically result in at least 1 "child" account (the adult user). There would be a new child table column that flags the child account as 'adult'. For this to work, we would either need the user to enter some additional information about themselves (birthday, gender, languages) to match the info needed for child accounts, or we would need to allow these fields to be null when the 'adult' column is True.

The downside of this option is that this breaks the semantics of the 'child' model (it really becomes 'participant', with two pretty distinct types). The benefit is that we would need to make fewer modifications to the rest of the system because we can continue to assume that each response is associated with a child object.

**With both options**

We would need to add a new boolean field to the study model/table and study create/edit form that flags the study as being only for adults.

Start by reading the response and child models/tables, then trace the study model and study create/edit form. Review experimenter pages, the family-facing study list, and the study announcement email template for child assumptions. Done means adult-only studies can represent and display adult participants across these areas, with tests covering the behavior.

Written by the indexing model from the issue text.