message: Prepare changelog for v0.12.1
diff: +Changes in [0.12.1](https://github.com/matrix-org/matrix-react-sdk/releases/tag/v0.12.1) (2018-04-11)
  +=====================================================================================================
  +[Full Changelog](https://github.com/matrix-org/matrix-react-sdk/compare/v0.12.0...v0.12.1)
  +
  +
  Changes in [0.12.0](https://github.com/matrix-org/matrix-react-sdk/releases/tag/v0.12.0) (2018-04-11)
  =====================================================================================================
  [Full Changelog](https://github.com/matrix-org/matrix-react-sdk/compare/v0.12.0-rc.7...v0.12.0)
  