Migrate services to NestJS

[lamkeewei]

After migrating sequelize in #1006, we will continue to make use of Nest in standalone application mode to instantiate services. Existing tests should be preserved and used to prevent regressions.

At the same time, models should passed directly to services instead of passing in sequelize.

Services to be migrated:

• AuthService
• CheckerService
• TemplateService
• Mailer
• TotpGenerator
• Logger

Example of how this would look like in boostrap:

const app = await NestFactory.createApplicationContext(AppModule)
...
const checker = new CheckerController({ service: app.get(CheckerService) })