refactor: Simplify accessing mongoose models

[jrassa]

Currently mongoose models are accessed via db connection and require a typecast.

import { UserModel } from './user.model';

const User = dbs.admin.model('User') as UserModel;

Since model registry is tied to the db connection, there is no need to access it in this way. We can just import the model (w/ proper typing) directly since it is already being exported.

import { User } from './user.model';

For use cases where multiple db connections are used. Models should explicitly be registered on the appropriate connection.

// Register model on default/admin db connection
export const MyModel = model('MyModel', MyModelSchema);

// Register model on secondary db connection
export const OtherModel = dbs.other.model('OtherModel', OtherModelSchema);