Update site host and is_multisite detection

[mdrovdahl]

Refactored eval_which_host() to improve hosting provider detection:

• adds explicit support for detecting WP VIP via .well-known/hosting-provider
• adds support for detecting WP Engine via the X-Powered-By http header
• improves detection of WP.com Simple
• improves detection of multisite (parent and sub sites)
• adds wp.com site-profiler API endpoint as a fallback source for hosting provider info

[mdrovdahl]

@cagrimmett not sure what our process is for moving this forward...I've tested it fairly extensively, but would be good to have a developer review the PR and correct any obvious issues.