Changelog entry for service class instrumentation

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CHANGELOG

 Please view this file on the master branch, on stable branches it's out of date.
 v 8.7.0 (unreleased)
+  - All service classes (those residing in app/services) are now instrumented (Yorick Peterse)
   - Enable gzip for assets, makes the page size significantly smaller. !3544 / !3632 (Connor Shea)
   - Load award emoji images separately unless opening the full picker. Saves several hundred KBs of data for most pages. (Connor Shea)
   - All images in discussions and wikis now link to their source files !3464 (Connor Shea).