[Backend] Implement the profile update API

[ege-kaya]

We need to implement the backend API allowing a user to update their own profile. We should first write the API documentation to the wiki page before starting the implementation, so that others can review it and the other subteams can start their implementations without needing the backend to be complete.

[ege-kaya]

I have improved the profile view controller to inherit the Django Rest Framework's RetrieveAPIView class, resulting in a better and cleaner implementation. I have also implemented the profile update view, but as of right now it does not check whether the request for the update comes from the correct user. I will now look into the user identification and authorization using the JWT token architecture we set up.

[ege-kaya]

I have implemented the user identification and permission-checking part of the profile update API as well. I have pushed my work to the improved-users-profiles branch, will now open a pull request and assign a reviewer. Also, I will promptly add the relevant API documentation to the Wiki page.