Support OAuth2

[hbusul]

Since our authentication/authorization mechanism is not mature yet, I believe it is in our best interest to use OAuth2. I don't know this too much as well so I will explain what is my aim by this transition in English:

• Users should be able to log in via well-known identity providers such as Google, Facebook etc
• We also should be an identity provider so they can register without using Google, Facebook etc
• We need to support Federated Access, when a 3rd party application wants to use Kucukdev, user should not enter their kucukdev passwords to that application but instead use OAuth2.

Here is some introductory video:
https://www.youtube.com/watch?v=t18YB3xDfXI

Some example open-source identity provider
https://github.com/ory/hydra