Delete draft changes and patchsets

Adds ability to delete draft changes and patchsets that are not meant
or fit for code review. Deleting a draft patchset also deletes the
corresponding ref from the repository and decrements the next patch
set number for the change if necessary. Deleting a draft change
deletes all of its (draft) patchsets.

Change-Id: I04abeb67b64dd2366514e74d23f83066d409904e

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

@@ -30,6 +30,7 @@ public class ChangeDetail {
   protected boolean canAbandon;
   protected boolean canRestore;
   protected boolean canRevert;
+  protected boolean canDeleteDraft;
   protected Change change;
   protected boolean starred;
   protected List<ChangeInfo> dependsOn;
@@ -94,6 +95,14 @@ public class ChangeDetail {
     canSubmit = a;
   }
 
+  public boolean canDeleteDraft() {
+    return canDeleteDraft;
+  }
+
+  public void setCanDeleteDraft(boolean a) {
+    canDeleteDraft = a;
+  }
+
   public Change getChange() {
     return change;
   }