message: Prepare changelog for v0.12.9-rc.2
diff: +Changes in [0.12.9-rc.2](https://github.com/matrix-org/matrix-react-sdk/releases/tag/v0.12.9-rc.2) (2018-07-06)
  +===============================================================================================================
  +[Full Changelog](https://github.com/matrix-org/matrix-react-sdk/compare/v0.12.9-rc.1...v0.12.9-rc.2)
  +
  + * Implement aggregation by error type for tracked decryption failures
  + [\#2045](https://github.com/matrix-org/matrix-react-sdk/pull/2045)
  + * make new hiding of roomsublist behaviour opt-in
  + [\#2044](https://github.com/matrix-org/matrix-react-sdk/pull/2044)
  + * Implement aggregation by error type for tracked decryption failures
  + [\#2043](https://github.com/matrix-org/matrix-react-sdk/pull/2043)
  + * make new hiding of roomsublist behaviour opt-in
  + [\#2030](https://github.com/matrix-org/matrix-react-sdk/pull/2030)
  +
  Changes in [0.12.9-rc.1](https://github.com/matrix-org/matrix-react-sdk/releases/tag/v0.12.9-rc.1) (2018-07-04)
  ===============================================================================================================
  [Full Changelog](https://github.com/matrix-org/matrix-react-sdk/compare/v0.12.8...v0.12.9-rc.1)
  