Email improvements

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# TL;DR

Some user requests from Tarun S. from an interview about the "all responses" page, but relevant to using the messaging system! Recording for use if/when we upgrade this part of the website.

- Being able to know more info about the participants you're seeing in the inbox would be very helpful - lots of toggling back and forth between two windows.
- - Did I pay this participant?
- - How long since their session?
- - NOTE: this is a little tough because the message system is helping you select *user IDs* from your list of participating families, but some of this information is actually at the session level. Should we attempt to associate each message with a SESSION? Show the most recent session but flag if multiple exist? Tricky design decision lurking here.
- Make it easier to see the original recruting message along with the others together in the inbox
- Sort or search messages by which are transactional, new study, feedback
- User frequently relies on date/time for quickly finding pieces of info, don't remove that!
- Message window is small/cramped, wishing for an option to expand it.
- Wants to use CHILD ID (not user ID) to search previous messages (same problem as above, what if multiple kids in one family have been in a study?)
- Group email from the "all responses" page - click on a bunch of sessions, then batch email all those people.

Start by reviewing the “all responses” page and messaging system workflows, especially the inbox and participant/session relationships. Clarify which requests belong in one scoped change and define acceptance criteria for the selected workflow before locating the relevant API and UI entry points. Done should be measurable against those agreed criteria.

Written by the indexing model from the issue text.