Update CHANGELOG

Moving 8.5 change to 8.4 since we're merging master into stable for now.

[ci skip]

CHANGELOG

 Please view this file on the master branch, on stable branches it's out of date.
-v 8.5.0 (unreleased)
-  - Remove gray background from layout in UI
-
 v 8.4.0 (unreleased)
   - Add pagination headers to already paginated API resources
   - Properly generate diff of orphan commits, like the first commit in a repository
@@ -19,6 +16,7 @@ v 8.4.0 (unreleased)
   - Fix missing date of month in network graph when commits span a month (Stan Hu)
   - Expire view caches when application settings change (e.g. Gravatar disabled) (Stan Hu)
   - Don't notify users twice if they are both project watchers and subscribers (Stan Hu)
+  - Remove gray background from layout in UI
   - Implement new UI for group page
   - Implement search inside emoji picker
   - Add API support for looking up a user by username (Stan Hu)