#162880221 Implement sign in functionality

[valenstical]

What does this PR do? This PR connects the frontend sign-in form to the backend sign-in api endpoint

Description of tasks

• Use vanilla Javascript Fetch API to make requests to POST / api/v1/auth/login endpoint
• Create a custom HTML DOM manipulation library to reduce code duplication
• Use Javascript localStorage to store authentication token and user data
• Refactor server-side code
• Rename frontend pages for easy identification

How can this PR be tested

• Visit https://valenstical.github.io/ireporter/sign-in.html
• Enter your username and password you used when signining up or use the default account below to sign in username: demo password: 123
• Press the sign in button

What are the relevant pivotal tracker stories? #162880221

Screenshot N/A