message: Prepare changelog for v0.12.0-rc.7
diff: +Changes in [0.12.0-rc.7](https://github.com/matrix-org/matrix-react-sdk/releases/tag/v0.12.0-rc.7) (2018-04-10)
  +===============================================================================================================
  +[Full Changelog](https://github.com/matrix-org/matrix-react-sdk/compare/v0.12.0-rc.6...v0.12.0-rc.7)
  +
  + * Reword group setting delay
  + [\#1816](https://github.com/matrix-org/matrix-react-sdk/pull/1816)
  + * Improve group joining/leaving feedback
  + [\#1831](https://github.com/matrix-org/matrix-react-sdk/pull/1831)
  +
  Changes in [0.12.0-rc.6](https://github.com/matrix-org/matrix-react-sdk/releases/tag/v0.12.0-rc.6) (2018-04-09)
  ===============================================================================================================
  [Full Changelog](https://github.com/matrix-org/matrix-react-sdk/compare/v0.12.0-rc.5...v0.12.0-rc.6)
  