MM-11160 Adding proper CORS support. (#9152)

* Adding proper CORS support.

* Better CORS tests.

config/default.json

@@ -33,6 +33,9 @@
         "EnforceMultifactorAuthentication": false,
         "EnableUserAccessTokens": false,
         "AllowCorsFrom": "",
+        "CorsExposedHeaders": "",
+        "CorsAllowCredentials": false,
+        "CorsDebug": false,
         "AllowCookiesForSubdomains": false,
         "SessionLengthWebInDays": 30,
         "SessionLengthMobileInDays": 30,