Add post-installation redirect based on admin account status

[kerwin612]

This PR adds a feature to direct users to appropriate pages after system installation:

• If no admin credentials were provided during installation, redirect to the registration page with a prominent notice about creating the first administrative account
• If admin credentials were already set, redirect directly to the login page

[kemzeb]

Can't speak on frontend design, but otherwise this looks good to me.

Was iffy about the HasUsers() calls ignoring DB errors, but after further reading it seems the worst that can happen is:

• User accesses sign up page (or completes the install config if they're on the install page)
• A DB error occurs
• Server sends sign up page + the sign_up_tip message to the user