message: Prepare changelog for v0.12.3
diff: +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)
  