Revamp authentication (#831)

* Implement Transport classes

* Implement authentication strategy classes

* Revamp authentication with Transport and Strategy

* Revamp strategy and OAuth so that they can use a callable dependency

* Update docstring

* Make ErrorCode a proper Enum and cleanup unused OpenAPI utils

* Remove useless check

* Tweak typing in authenticator

* Update docs

* Improve logout/destroy token logic

* Update docs

* Update docs

* Update docs and full examples

* Apply formatting to examples

* Update OAuth doc and examples

* Add migration doc

* Implement Redis session token

* Add Redis Session documentation

* RedisSession -> Redis

* Fix links in docs

examples/mongodb/sqlalchemy/app/models.py

@@ -0,0 +1,17 @@
+from fastapi_users import models
+
+
+class User(models.BaseUser):
+    pass
+
+
+class UserCreate(models.BaseUserCreate):
+    pass
+
+
+class UserUpdate(models.BaseUserUpdate):
+    pass
+
+
+class UserDB(User, models.BaseUserDB):
+    pass