add core app

[ahmed-arb]

What?

Add core app and user authentication. App include; book, loan, and author models and their migrations, views, and URLs.

Why?

These changes complete the basic library CRUD and authentication.

How

User authentication is implemented using Djoser and simplejwt.

[abdullahwaheed]

overall pretty good work. Here are some suggestion:

• Please take a look at pylint. its a linter for python. in the future assignments, we will be imposing pylint to make sure our code meet pep-8 style guide for python
• PR title should be concise and descriptive and you should add a short PR description describing what the PR is about.
• Commit your code whenever you add a functionality. e.g. when you have setup project for the first time, that should be first commit. When you added models and migration, that should be second commit.
• Commit message should be meaningful and should follow these conventions