Since the session cache clears keys after a period of time, this fixes#1821.
Based on https://github.com/mail-in-a-box/mailinabox/pull/2012, and so:
Co-Authored-By: NewbieOrange <NewbieOrange@users.noreply.github.com>
Also fixes#2029 by not revealing through the login failure error message whether a user exists or not.