Migrate database access to NestJS

[lamkeewei]

The first step of the migration involves moving Sequelize into Nest. Existing model definitions can be reused as we already make use of sequelize-typescript. We will make use of Nest in standalone application mode to initialise and pass models to services and controllers. Tests should continue to pass.

Models to be migrated:

• Checker
• PublishedChecker
• Template
• User
• UserToChecker

Example of how bootstrap would be modified after this step:

const app = await NestFactory.createApplicationContext(AppModule)
...
const sequelize = app.get(Sequelize)