Add a CHANGELOG entry for The Most Important Feature of All Time(TM)

Also, Open Graph data, if you're into that kind of thing.

[ci skip]

CHANGELOG

 Please view this file on the master branch, on stable branches it's out of date.
 
 v 8.4.0 (unreleased)
+  - The default GitLab logo now acts as a loading indicator
   - Accept 2xx status codes for successful Web hook triggers (Stan Hu)
   - 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)
@@ -21,6 +22,7 @@ v 8.4.0 (unreleased)
   - Add system hook messages for project rename and transfer (Steve Norman)
   - Fix version check image in Safari
   - Show 'All' tab by default in the builds page
+  - Add Open Graph and Twitter Card data to all pages
   - Fix API project lookups when querying with a namespace with dots (Stan Hu)
   - Update version check images to use SVG
   - Validate README format before displaying