Auth: Increase minimum password length from 4 to 8 characters

[lastzero]

As a user connecting its instance to the public Internet, I want to set a reasonably long password to make brute force attacks more difficult.

An integer constant should be introduced to set the minimum length at compile time.

Related Issues:

• Sharing: Multi-user / multi-library support with private and shared photos/albums #98
• Auth: Add support for single sign-on via OpenID Connect (OIDC) #782