Remove reviewers from a change

- The new UI adds a "Remove" column and an "X" button on each
  reviewer row to ApprovalTable.  Clicking this button will cause
  a remote call to removeReviewer

- Added a new removeReviewer remote method to PatchDetailService

- Both the addReviewer and removeReviewer remote methods use the
  same result class, which I renamed from AddReviewerResult to
  ReviewerResult

- ReviewerResult received a new error type "COULD_NOT_REMOVE"

Note: the error handling of this CL is not optimal for 2 reasons:

- Since PatchDetailService always catches exceptions and adds them
  to an errors field in ReviewerResult, the call always succeeds
  even if it actually failed.

- We can display a dialog box if the removal failed, but it would
  be better simply not to display the Delete button if the user
  doesn't have enough privileges.

A cleaner way to implement this (future change list) would be to
return this information in ChangeDetail so that ChangeScreen will
only display Delete buttons that will actually work.

Bug: issue 218
Bug: issue 289
Change-Id: I38ce63d76a3588c85472f1fd723cd097aeafccb3

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

@@ -50,7 +50,11 @@ public interface PatchDetailService extends RemoteJsonService {
 
   @SignInRequired
   void addReviewers(Change.Id id, List<String> reviewers,
-      AsyncCallback<AddReviewerResult> callback);
+      AsyncCallback<ReviewerResult> callback);
+
+  @SignInRequired
+  void removeReviewer(Change.Id id, Account.Id reviewerId,
+      AsyncCallback<ReviewerResult> callback);
 
   void userApprovals(Set<Change.Id> cids, Account.Id aid,
       AsyncCallback<ApprovalSummarySet> callback);

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

@@ -18,12 +18,14 @@ package com.google.gerrit.common.data;
 import java.util.ArrayList;
 import java.util.List;
 
-/** Result from adding a reviewer to a change. */
-public class AddReviewerResult {
+/**
+ * Result from adding or removing a reviewer from a change.
+ */
+public class ReviewerResult {
   protected List<Error> errors;
   protected ChangeDetail change;
 
-  public AddReviewerResult() {
+  public ReviewerResult() {
     errors = new ArrayList<Error>();
   }
 
@@ -49,7 +51,10 @@ public class AddReviewerResult {
       ACCOUNT_NOT_FOUND,
 
       /** The account is not permitted to see the change. */
-      CHANGE_NOT_VISIBLE
+      CHANGE_NOT_VISIBLE,
+
+      /** Could not remove this reviewer from the change. */
+      COULD_NOT_REMOVE
     }
 
     protected Type type;