Add CLA check to ProjectControl.canPushToAtLeastOneRef()

This is a refactoring, it should not change functionality
at all.  Move CLA check out of ReceiveCommits and into
ProjectControl.  Create a common Capable class to facilitate
this.

Change-Id: I43ff798bcb727918eb39b1ec8dc7e14278c91b1f

gerrit-common/src/main/java/com/google/gerrit/common/data/Capable.java

@@ -0,0 +1,29 @@
+// Copyright (C) 2008 The Android Open Source Project
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package com.google.gerrit.common.data;
+
+public class Capable {
+  public static final Capable OK = new Capable("OK");
+
+  private final String message;
+
+  public Capable(String msg) {
+    message = msg;
+  }
+
+  public String getMessage() {
+    return message;
+  }
+}