Update CHANGELOG and VERSION for 8.6.0-pre

CHANGELOG

 Please view this file on the master branch, on stable branches it's out of date.
 
-v 8.5.0 (unreleased)
+v 8.6.0 (unreleased)
+
+v 8.5.0
   - Fix duplicate "me" in tooltip of the "thumbsup" awards Emoji (Stan Hu)
   - Cache various Repository methods to improve performance (Yorick Peterse)
   - Fix duplicated branch creation/deletion Web hooks/service notifications when using Web UI (Stan Hu)

VERSION

-8.5.0-pre
+8.6.0-pre