Switch postgresql user management to flask-login

[shreystechtips]

Change away from a ground-up approach for user management in the API, since the Flask-Login package can use @is_autheticated decorators which are useful in rendering new pages when users want to try out the API without using a client or craft their own POST requests.

Also, I will be able to display the API key on more pages than just rendering the login page with the API Key, which looks not so nice. I can have a 'user management' page where users can create/delete API Keys (and not be locked to one api key per account).

[shreystechtips]

idk if its worth doing...