Revamp authentication routes structure (#201)

* Fix #68: use makefun to generate dynamic dependencies

* Remove every Starlette imports

* Split every routers and remove event handlers

* Make users router optional

* Pass after_update handler to get_users_router

* Update documentation

* Remove test file

* Write migration doc for splitted routers

fastapi_users/router/auth.py

@@ -0,0 +1,40 @@
+from fastapi import APIRouter, Depends, HTTPException, Response, status
+from fastapi.security import OAuth2PasswordRequestForm
+
+from fastapi_users import models
+from fastapi_users.authentication import Authenticator, BaseAuthentication
+from fastapi_users.db import BaseUserDatabase
+from fastapi_users.router.common import ErrorCode
+
+
+def get_auth_router(
+    backend: BaseAuthentication,
+    user_db: BaseUserDatabase[models.BaseUserDB],
+    authenticator: Authenticator,
+) -> APIRouter:
+    """Generate a router with login/logout routes for an authentication backend."""
+    router = APIRouter()
+
+    @router.post("/login")
+    async def login(
+        response: Response, credentials: OAuth2PasswordRequestForm = Depends()
+    ):
+        user = await user_db.authenticate(credentials)
+
+        if user is None or not user.is_active:
+            raise HTTPException(
+                status_code=status.HTTP_400_BAD_REQUEST,
+                detail=ErrorCode.LOGIN_BAD_CREDENTIALS,
+            )
+
+        return await backend.get_login_response(user, response)
+
+    if backend.logout:
+
+        @router.post("/logout")
+        async def logout(
+            response: Response, user=Depends(authenticator.get_current_active_user)
+        ):
+            return await backend.get_logout_response(user, response)
+
+    return router