Revert back to S3 html template logic in registration email process

🗣 Description

part of #2508 Reverts back to S3 html template logic for registration email assets.

Leadership request to revert back to S3 because local email asset mapping to lambda was causing errors in email functionality.

Registration should send email generated from s3 template assets.

[x] This PR has an informative and human-readable title.
[x] Changes are limited to a single goal - eschew scope creep!
[x] All future TODOs are captured in issues, which are referenced in code comments.
[x] All relevant type-of-change labels have been added.
[x] I have read the CONTRIBUTING document.
[x] These code changes follow cisagov code standards.
[x] All relevant repo and/or project documentation has been updated to reflect the changes in this PR.
[x] Tests have been added and/or modified to cover the changes in this PR.
[x] All new and existing tests pass.