@nest-modules/mailer integration changes

[ZenSoftware]

@nest-modules/mailer is unmaintained.

• [x] Evaluate @squareboat/nest-mailman as a substitute
  • @squareboat/nest-mailman uses Nest v7. View the npm install error:
  • npm ERR! peer @nestjs/common@"^6.7.0 || ^7.0.0" from @squareboat/nest-mailman@0.3.0
  • @nest-modules/mailer is on Nest v9. It is better to keep this dependency
• [x] Upgrade to @nestjs-modules/mailer. They changed their npm package name. Re-integrate with the latest version.
  • Finished with: https://github.com/ZenSoftware/zen/commit/b91dedc503679cf623ec5ad4b334d1bb6d94487c
• [x] Restructure dev ops to utilize /assets/mail as the directory to house .hbs mail templates.