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