practice-app/bug-fix/user-#87 User API bugs are fixed

[mehmetemreakbulut]

After implementing essential functionalities of User API, some bugs has arisen.

• Bugs are listed in the issue Practice App: Implementing API for User - Bug Fix and Enhancements #87.

Bugs that is fixed:

• No error message as response to the creation of a user with username that already exists.
• Can't reach data when POST request is sended as a row JSON from Postman.
• Allows password field to be empty
• Don't check whether email is taken or not
• No error message if user creation is failed
• Email is not mandatory currently, however email is required for external API.
• No email validation

Also endpoints are changed.

• New endpoint for listing all users and creating a user: user/api.
• New endpoint for showing a spesific user detail with given username: user/api/<str: username>.
• Structure of APIs is same as mentioned in issue Practice App: Implementing API for User #81.

After review, we can merge to master.