Add Changelog entry for preserving timestamps when moving issue

CHANGELOG

 Please view this file on the master branch, on stable branches it's out of date.
 
 v 8.7.0 (unreleased)
+  - Preserve time notes/comments have been updated at when moving issue
   - Make HTTP(s) label consistent on clone bar (Stan Hu)
 
 v 8.6.1 (unreleased)