Centralized API and Tests

[jinyung2]

Create central location for axios API calls on the authentication and user side, write Tests for api calls

• used axios.create and interceptors for request and response
• created separate axios api objects for authentication and user (where tokens were needed)
• wrote unit tests for all api calls
• replaced axios function calls in all files with newly created functions within Auth.js and User.js
• organized to be modular and more easily scalable :smile_cat:

closes #36