feat: route account activation emails via SES using waffle flag

[nthriveni-sonata-ship-it]

This PR introduces a waffle-flag–controlled routing for Account Activation emails to be sent via SES.

What’s included:

Added a waffle flag to control SES routing for account activation emails

Updated activation email composition to override the default ACE channel to django_email when the flag is enabled

Local devstack validation using a file-based email backend to verify channel override and email generation

Testing:

Verified waffle flag behavior locally via Django admin

Confirmed ACE message options include transactional and django_email channel override

Validated rendered email output using the file-based email backend

Next steps:
– Draft PR for early review and alignment

[asharma4-sonata]

Hi @nthriveni-sonata-ship-it, couple of suggestions on refactor:

1. Centralize the Routing Logic
   Currently, the SES routing logic is tied specifically to the management view. To ensure it also works for initial registrations (via Celery tasks) and user-requested resends, we should pull the logic into a centralized utility.
   Action: Move the WaffleFlag definitions and the check logic to: openedx/core/djangoapps/ace_common/utils.py

2. Implement an Active Fallback (Safe-to-Fail) in openedx/core/djangoapps/user_authn/tasks.py
   The current implementation introduces a single point of failure. If the Waffle flag is ON but the SES backend (django_email) is unreachable, the email will never be sent. We should prioritize SES but fallback to the default ACE path if it fails.

Why?
One function handles routing for all activation/course-update flows.
Even if SES has an outage, user registrations won't be blocked.
Management views and Celery tasks don't need to know the names of individual Waffle flags.