#164198181 Validates signup parameters

[valenstical]

What does this PR do?
This PR validates registration/signup parameters before sending control over to the user controller.

Description of Task to be completed

• Install express-validator to handle validation
• Create middleware to validate each parameter
• Create middleware to handle validation errors if any
• Mount middleware created above to the user signup route
• Write test against implementation
• Create helper function to send response data
• Create constant file to hold all re-usable constants

How should this be manually tested

Prerequisite: Postgres

1. Run the following commands on terminal:

git clone -b feature/164198181/validate-signup --single-branch \
https://github.com/andela/apollo-ah-backend.git

Navigate to the apollo-ah-backend directory
Run cp .env.example .env
npm install

2. Modify .env file with your db credentials

3. Run migrations with npm run migrate

4. Spin up the server by running npm run start:dev on your terminal

5. Make a POST request to localhost:3000/api/v1/users respectively with Postman.

❗️ Important: Provide user payload when making request, sample below:

{
  "email": "example@email.com",
  "password": "secret"
  "username" "user"
}

NOTE: To run test locally, create .env.test file. Run command: cp .env.example .env.test

Any background context you want to provide

N/A

What are the relevant pivotal tracker stories

#164198181

Screenshots

N/A

-- Template format by @devdbrandy

[aanchirinah]

Great job done @valenstical , kindly attend to my requested changes

[devdbrandy]

Looks good to me.

[aanchirinah]

LGTM