Improve test coverage using mocks and stubs

[chidimo]

What does this PR do?

Improve test coverage using mocks and stubs

Description of Task to be completed?

1. Mock out internal workings of email sender, generateToken, and verifyToken

How should this be manually tested?

1. Clone the repo
2. cd into the root folder on a cmd shell and run yarn install. PowerShell is fine also.
3. Setup a PostgreSQL DB.
4. Define a .env file in the root of the project and supply values for the following parameters

   JWT_SECRET=<json-web-token-secret>
   DBNAME=<database-name>
   PGPASSWORD=<postgres-db-password>
   PGHOST=<postgres-db-host>
   PGUSER=<postgres-db-user>
   PGPORT=<postgres-db-port>

5. Create a PostgreSQL database called testdb.
6. Run command yarn test

Any background context you want to provide?

Nil

What are the relevant pivotal tracker stories?

166322948

Screenshots (if appropriate)

Nil

Questions:

Nil