message: Prepare changelog for v0.12.0
diff: +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)
  +
  + * Further improve group joining/leaving feedback
  + [\#1832](https://github.com/matrix-org/matrix-react-sdk/pull/1832)
  + * Cosmetic changes to Communities button
  +
  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)
  