Prepare changelog for v0.12.3

Author: dbkr

CHANGELOG.md

@@ -1,3 +1,9 @@
+Changes in [0.12.3](https://github.com/matrix-org/matrix-react-sdk/releases/tag/v0.12.3) (2018-04-30)
+=====================================================================================================
+[Full Changelog](https://github.com/matrix-org/matrix-react-sdk/compare/v0.12.3-rc.3...v0.12.3)
+
+ * No changes since rc.3
+
 Changes in [0.12.3-rc.3](https://github.com/matrix-org/matrix-react-sdk/releases/tag/v0.12.3-rc.3) (2018-04-26)
 ===============================================================================================================
 [Full Changelog](https://github.com/matrix-org/matrix-react-sdk/compare/v0.12.3-rc.2...v0.12.3-rc.3)