Add patch set description

With this change, the description field is properly populated by the %m
flag and set in ReviewDB and NoteDB, and is set in the ChangeJSON
received in a GetChange call. The test for pushing a change with a
message was also updated to check for the description field being set in
RevisionInfo.

To be included in descendant changes:
 - Handling cases where PSUtil.insert is called that aren't change
   creations or updates (cherrypick, etc)
 - Implementation of an API endpoint for setting/deleting the value
 - Docs

Feature: Issue 4544
Change-Id: I1c4d817d1d5e263b0cd42bf9ea874808aac00582

gerrit-extension-api/src/main/java/com/google/gerrit/extensions/common/RevisionInfo.java

@@ -33,4 +33,5 @@ public class RevisionInfo {
   public Map<String, ActionInfo> actions;
   public String commitWithFooters;
   public PushCertificateInfo pushCertificate;
+  public String description;
 }