Add an antilockout user

[signebedi]

When auth is enabled, we should support an antilockout user. See eg. initial discussion in config.yaml:

# this application is intended to be run using external auth providers. Yes, you 
# could probably hack it to support a more robust form of local authentication. 
# But then things might start to act erratically. Instead, you should configure
# an antilockout user here when you need to access the system in emergencies.
# Doing so will allow you to login at YOUR_DOMAIN_HERE/lockout. You should then
# use the username/password: `wikiflask/lockedout`.
enable_antilockout_user: false