Hoist getUserName up onto CurrentUser

Since all users have a unique username on the server, this makes
it cleaner to determine who is who.

Change-Id: Ifc97f3c3a5b469182beb95fdc2689e226cc38181

gerrit-server/src/main/java/com/google/gerrit/server/CurrentUser.java

@@ -68,6 +68,11 @@ public abstract class CurrentUser {
   /** Filters selecting changes the user wants to monitor. */
   public abstract Collection<AccountProjectWatch> getNotificationFilters();
 
+  /** Unique name of the user on this server, if one has been assigned. */
+  public String getUserName() {
+    return null;
+  }
+
   /** Capabilities available to this user account. */
   public CapabilityControl getCapabilities() {
     CapabilityControl ctl = capabilities;