Replace with @nestjs-modules/mailer with plain nodemailer

[vygandas]

Problem

@nestjs-modules/mailer causes many security vulnerabilities.

Solution

We do not want this project to be vulnerable, right?
Here's a proposed solution
nest-modules/mailer#1074 (comment)

Acceptance criteria

• We can still use .hpb templates. When writing this issue, the template email-confirmation.hbs is sent on registration.
• It should be easy to send emails (easy for developers)
• It should have its own documentation file in docs/ dir and link to it on the main Readme