Add resave command for all HistoryEntries

This pattern will mainly be used for data migrations, i.e. updating all
HistoryEntries' DomainTransactionRecords to the new schema.

TESTED=Ran in alpha, the underlying data dropped non-Objectify fields as
expected.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=168684356

java/google/registry/env/common/tools/WEB-INF/web.xml

@@ -72,6 +72,12 @@
     <url-pattern>/_dr/task/resaveAllEppResources</url-pattern>
   </servlet-mapping>
 
+  <!--  Mapreduce to re-save all HistoryEntries. -->
+  <servlet-mapping>
+    <servlet-name>tools-servlet</servlet-name>
+    <url-pattern>/_dr/task/resaveAllHistoryEntries</url-pattern>
+  </servlet-mapping>
+
   <!-- Mapreduce to delete EppResources, children, and indices. -->
   <servlet-mapping>
     <servlet-name>tools-servlet</servlet-name>