Provide proper validation feedback during setup

[mvz]

Fixes publify/publify#1142.

• Ensure blog is not set up if parameters are invalid
• Provide feedback about weak password during setup
• Keep entered values when there is a validation error during setup
• Don't needlessly set autodiscovery feeds if blog is not configured
• Avoid each sidebar loading the blog during seeding
• Clear blog name in first setup form
• Provide feedback for missing blog name during setup