[BB-8672] feat: 'From user' for emails.

This MR allows setting a specific team member to be selected for the purposes of sending reminder emails.

Supporting information

• Jira ticket: url
• GitLab issue: url

Testing instructions

Step by step procedure

1. Check out this branch and run migrations
2. Set up recurrence in the admin with a short interval, like 5 minutes, and a same day due date with default reminder.
3. Wait a bit for a recurrence to generate.
4. In one terminal tab, make logs
5. In another, make shell
6. then ./manage.py shell and

   from workflow.tasks import send_reminder_emails
   send_reminder_emails()

7. Check the headers of the email to make sure they make sense-- showing the email to be labeled as from their team rather than Listaflow, but with Listaflow's email.
8. Go back to the recurrence in the admin and pick a team member as the 'from_user`. Save, and send reminders again.
9. Verify the resulting headers show the selected person as the sender.

Author notes and concerns

I'm removing the reply_to field on email templates. Let me know if you see any issues with this.