Enforce password validation rules on system admins

[andreslucena]

🎩 What? Why?

As mentioned in #9090, password validation rules aren't enforced for system admins. This PR fixes that.

📌 Related Issues

• Related to Enforce password validation rules on 'Forgot your password?' form #9090

Testing

For new system users with the rake task:

1. Run bin/rails decidim_system:create_admin
2. Fill email "system2@example.org"
3. Fill password and password confirmation "password1234".
4. See "password is too common" error

For new system users with the rails console:

1. bin/rails console
2. Decidim::System::Admin.create!(email: "system2@example.org", password: "password1234", password_confirmation: "password1234")
3. See "password is too common" error

For existing users through the system panel:

1. Log in to /system
2. Go to Admins
3. Edit an admin
4. Fill password and password confirmation "password1234".
5. See "password is too common" error

📷 Screenshots

♥️ Thank you!