Automate volunteer user surveys

### Description:
To automate user surveys by integrating Mailchimp API into DemocracyLab’s codebase

### Tasks:
1. On-boarding Feedback Survey - Survey sent after 2 weeks of approval date.
2. Month Two Check-in Survey - Survey sent after 6 weeks of approval date.
3. Check-in Survey for Existing Volunteers - Survey sent to existing active volunteers.
4. Volunteer Commitment Renewal Survey - Survey sent to volunteers renewing their contract.
5. Exit Survey - Survey sent to volunteers whose commitment has ended.

### Steps
1) Check if existing code base supports automated surveys or check MailChimp API to be integrated
more info on Mailchimp - https://mailchimp.com/developer/tools/

2) Identify correct way to get volunteers for all 5 tasks from the database.
SQL queries/Triggers - contact Pruthvi for more info

3) Test with different data

### Extra
Check existing automated emails sent