Restrict file view

[erincerys]

As the purpose of the app is to expose intentionally shared media, it doesn't make sense to me that it will respond to arbitrary /view/<hash> requests.

I would propose an option to restrict file viewing by the share namespace; that is, when the /view API endpoint is hit for a piece of media, and this new option is enabled, the tag mappings for said media would be checked for the presence of the sharing namespace and, if non-mapped, an HTTP 403 would be returned.