message: Prepare changelog for v0.12.8
diff: +Changes in [0.12.8](https://github.com/matrix-org/matrix-react-sdk/releases/tag/v0.12.8) (2018-06-29)
  +=====================================================================================================
  +[Full Changelog](https://github.com/matrix-org/matrix-react-sdk/compare/v0.12.8-rc.2...v0.12.8)
  +
  + * Revert "affix copyButton so that it doesn't get scrolled horizontally"
  + [\#2013](https://github.com/matrix-org/matrix-react-sdk/pull/2013)
  + * don't fire share dialog when clicking timestamp of event
  + [\#2017](https://github.com/matrix-org/matrix-react-sdk/pull/2017)
  + * when the user switches room, close room settings
  + [\#2019](https://github.com/matrix-org/matrix-react-sdk/pull/2019)
  +
  Changes in [0.12.8-rc.2](https://github.com/matrix-org/matrix-react-sdk/releases/tag/v0.12.8-rc.2) (2018-06-22)
  ===============================================================================================================
  [Full Changelog](https://github.com/matrix-org/matrix-react-sdk/compare/v0.12.8-rc.1...v0.12.8-rc.2)
  