Scripts should give a "you are signed up" page instead of 403 forbidden

[ezyang]

The standard case for someone signing up for Scripts is that their web_scripts directory is empty, which results in a 403 since we don't enable directory listing by default. Instead, display a pretty page telling the user they have successfully signed up for our service.

[achernya]

It seems reasonable to drop a stock index.html with instructions that the user should replace this file with their own content upon signup. This has the obvious limitations that existing accounts will continue to show a 403 error.