message: Prepare changelog for v0.12.4-rc.5
diff: +Changes in [0.12.4-rc.5](https://github.com/matrix-org/matrix-react-sdk/releases/tag/v0.12.4-rc.5) (2018-05-15)
  +===============================================================================================================
  +[Full Changelog](https://github.com/matrix-org/matrix-react-sdk/compare/v0.12.4-rc.4...v0.12.4-rc.5)
  +
  + * Wait for echo from server when adding user widgets
  + [\#1905](https://github.com/matrix-org/matrix-react-sdk/pull/1905)
  +
  Changes in [0.12.4-rc.4](https://github.com/matrix-org/matrix-react-sdk/releases/tag/v0.12.4-rc.4) (2018-05-14)
  ===============================================================================================================
  [Full Changelog](https://github.com/matrix-org/matrix-react-sdk/compare/v0.12.4-rc.3...v0.12.4-rc.4)
  