Rewrite /avatar/{id} as REST API /accounts/{id}/avatar

Implement avatar lookup as a REST API. GET on the /avatar
URL will return the avatar as supplied by the avatar plugin.

Change-Id: Ieb581558a8b8f7863dcb43ecea39b016dbdc696e

gerrit-server/src/main/java/com/google/gerrit/server/account/Module.java

@@ -29,6 +29,7 @@ public class Module extends RestApiModule {
     DynamicMap.mapOf(binder(), ACCOUNT_KIND);
     DynamicMap.mapOf(binder(), CAPABILITY_KIND);
 
+    get(ACCOUNT_KIND, "avatar").to(GetAvatar.class);
     child(ACCOUNT_KIND, "capabilities").to(Capabilities.class);
     get(ACCOUNT_KIND, "groups").to(GetGroups.class);
     get(CAPABILITY_KIND).to(GetCapabilities.CheckOne.class);