Add notification filtering by target media type

The Hub registry generates a large volume of notifications, many of
which are uninteresting based on target media type.  Discarding them
within the notification endpoint consumes considerable resources that
could be saved by discarding them within the registry.  To that end,
this change adds registry configuration options to restrict the
notifications sent to an endpoint based on target media type.

Signed-off-by: Noah Treuhaft <noah.treuhaft@docker.com>

configuration/configuration_test.go

@@ -59,6 +59,7 @@ var configStruct = Configuration{
 				Headers: http.Header{
 					"Authorization": []string{"Bearer <example>"},
 				},
+				IgnoredMediaTypes: []string{"application/octet-stream"},
 			},
 		},
 	},
@@ -136,6 +137,8 @@ notifications:
       url:  http://example.com
       headers:
         Authorization: [Bearer <example>]
+      ignoredmediatypes:
+        - application/octet-stream
 reporting:
   bugsnag:
     apikey: BugsnagApiKey
@@ -162,6 +165,8 @@ notifications:
       url:  http://example.com
       headers:
         Authorization: [Bearer <example>]
+      ignoredmediatypes:
+        - application/octet-stream
 http:
   headers:
     X-Content-Type-Options: [nosniff]