Return 404 for resources in sandbox/ js/ fonts/ images/ that don't exist

[qq99]

We handle all other URLs as requests to join/create a channel
But we know that anything with a base of:

• /sandbox/*
• /js/*
• /images/*
• /fonts/*

can be resolved to an actual file resource. We can be bit smarter and prevent at least false positive dev errors by returning 404 instead of simply serving up index.dev.html with 200 when it doesn't match a static file