Use Login with Lichess OAuth API

[tomsihap]

Feature: Login with Lichess OAuth API
  Scenario: User creates a Listudy account
    Given the User has a Lichess account
    When the User creates an account
    Then the User can create an account by clicking "Register with Lichess" button
  
  Scenario: User already has a Listudy account
    Given the User has a Lichess account
    When the User clicks "Connect with Lichess" in the settings
    Then the User can merge its Lichess account with "Login with Lichess" button

See:

• Authorization Code Flow with PKCE
• Real life example: Rotherham Online Chess

It will permit Listudy to work with Lichess API flawlessly since User is authenticated, and may be the first step to add future features around Lichess API.

Child: #90

[ArneVogel]

Thanks for the suggestion. This is something I considered when contemplating on authentication methods. I decided against it since maintaining multiple authentication methods seemed more trouble than it was worth it. But will keep this open as a enhancement.