Add basic text and html email logic to user registration

[nickviola]

🗣 Description

Adding new email functions for basic testing of text and html formats.
part of #2380

💭 Motivation and context

Testing User registration email notification process.

🧪 Testing

Verify email is sent on registration signup.

✅ Pre-approval checklist

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

✅ Pre-merge checklist

• Revert dependencies to default branches.
• Finalize version.

✅ Post-merge checklist

• Create a release.

[nickviola]

Vulnerability failures and censys 401 test failure will be addressed in separate PRs.

[schmelz21]

Approve to run email testing.