Use compliance modes throughout

[lucacasonato]

Use the compliance modes from #19 throughout the spec:

• CORS
• Cookies
• Caching
• Service workers
• Basic auth popup

[andreubotella]

It seems like a "supports HTTP cache" compliance mode would indeed be needed, since the "HTTP-network-or-cache fetch" algorithm can add Cache-Control and Pragma request headers, as well as return a network error for uncached responses, depending on the request's cache option. None of which seems acceptable for implementations without an HTTP cache.