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>

docs/configuration.md

@@ -212,6 +212,8 @@ information about each option that appears later in this page.
           timeout: 500
           threshold: 5
           backoff: 1000
+          ignoredmediatypes:
+            - application/octet-stream
     redis:
       addr: localhost:6379
       password: asecret
@@ -1162,6 +1164,8 @@ settings for the registry.
           timeout: 500
           threshold: 5
           backoff: 1000
+          ignoredmediatypes:
+            - application/octet-stream
 
 The notifications option is **optional** and currently may contain a single
 option, `endpoints`.
@@ -1276,6 +1280,18 @@ The URL to which events should be published.
     If you omit the suffix, the system interprets the value as nanoseconds.
     </td>
   </tr>
+  <tr>
+    <td>
+      <code>ignoredmediatypes</code>
+    </td>
+    <td>
+      no
+    </td>
+    <td>
+      List of target media types to ignore. An event whose target media type
+      is present in this list will not be published to the endpoint.
+    </td>
+  </tr>
 </table>