Refactor therapy feedback email

[annarhughes]

What changes did you make?

Refactors email functionality for existing automated emails send from bloom - the Impact measurement email and the therapy feedback email.

Replaces mandrill + mailchimp setup, with mailchimp automations. Bloom users are now added to mailchimp, with user fields and events updated on mailchimp to trigger automated email flows.

User account created -> create user in mailchimp with signed_up_at
User completes therapy session -> update mailchimp user first_therapy_session_at and last_therapy_session_at field

Mailchimp automation to trigger impact measurement email based on signed_up_at date
Mailchimp automation to trigger therapy feedback email based on first_therapy_session_at date

This PR completes the transition of existing emails for Bloom, however it is now much easier to send additional one-off and automated (transactional) emails using mailchimp user fields, and/or mailchimp events

Why did you make the changes?

We are introducing mailchimp automations for our transactional (user-triggered) emails across Chayn services. We previously used Mandrill as an add on for transactional emails, which is currently being integrated into mailchimp. However mailchimps existing automations now seems to be the best tool for most of our email requirements, including our Bloom feedback emails. So we’re moving the current functionality for bloom emails to automations.