Fixes server error on embed due to breaking change on flask-login

Due to a breaking change in Flask-Login (maxcountryman/flask-login#378) the code for logging in our the AnonymousUser breaks. Unfortunately, Flask-Login not only renames the method we need, but also makes it quasi-private. We can switch to a different public util function Flask-Login offers since at least version 0.3.0. In all versions I checked it essentially executes the same steps as `reload_user(...)` did (it additionally signals the login event internally, which shouldn't cause issues).

Fixes apache#21987

superset/embedded/view.py

@@ -19,10 +19,10 @@
 
 from flask import abort, g, request
 from flask_appbuilder import expose
-from flask_login import AnonymousUserMixin, LoginManager
+from flask_login import AnonymousUserMixin, login_user
 from flask_wtf.csrf import same_origin
 
-from superset import event_logger, is_feature_enabled, security_manager
+from superset import event_logger, is_feature_enabled
 from superset.embedded.dao import EmbeddedDAO
 from superset.superset_typing import FlaskResponse
 from superset.utils import core as utils
@@ -68,8 +68,7 @@ def embedded(
         # Log in as an anonymous user, just for this view.
         # This view needs to be visible to all users,
         # and building the page fails if g.user and/or ctx.user aren't present.
-        login_manager: LoginManager = security_manager.lm
-        login_manager.reload_user(AnonymousUserMixin())
+        login_user(AnonymousUserMixin(), force=True)
 
         add_extra_log_payload(
             embedded_dashboard_id=uuid,

superset/views/dashboard/views.py

@@ -24,7 +24,7 @@
 from flask_appbuilder.models.sqla.interface import SQLAInterface
 from flask_appbuilder.security.decorators import has_access
 from flask_babel import gettext as __, lazy_gettext as _
-from flask_login import AnonymousUserMixin, LoginManager
+from flask_login import AnonymousUserMixin, login_user
 
 from superset import db, event_logger, is_feature_enabled, security_manager
 from superset.constants import MODEL_VIEW_RW_METHOD_PERMISSION_MAP, RouteMethod
@@ -149,8 +149,7 @@ def embedded(
         # Log in as an anonymous user, just for this view.
         # This view needs to be visible to all users,
         # and building the page fails if g.user and/or ctx.user aren't present.
-        login_manager: LoginManager = security_manager.lm
-        login_manager.reload_user(AnonymousUserMixin())
+        login_user(AnonymousUserMixin(), force=True)
 
         add_extra_log_payload(
             dashboard_id=dashboard_id_or_slug,