Deactivate local auth on Zimfarm

[benoit74]

Now that it is possible to login Zimfarm with Ory, I will deactivate local authentication for user accounts (workers will continue to authenticate with "local" SSH keys):

• UI will not show anymore the possibility to login with local user/password
• API will not accept anymore auth token locally crafted
• I will delete all users passwords except for the backup admin account

[benoit74]

I've turned off UI capabilities to login with local user/password

We have other blocking issues before we can shutdown API usage of local username/password.