message: Prepare changelog for v0.12.4-rc.2
diff: +Changes in [0.12.4-rc.2](https://github.com/matrix-org/matrix-react-sdk/releases/tag/v0.12.4-rc.2) (2018-05-09)
  +===============================================================================================================
  +[Full Changelog](https://github.com/matrix-org/matrix-react-sdk/compare/v0.12.4-rc.1...v0.12.4-rc.2)
  +
  + * Improve appearance of short-lived widget loading spinner
  + * Make sticker picker fully-fledged feature
  + * Fix incorrect positioning with widget loading indicator
  +
  Changes in [0.12.4-rc.1](https://github.com/matrix-org/matrix-react-sdk/releases/tag/v0.12.4-rc.1) (2018-05-09)
  ===============================================================================================================
  [Full Changelog](https://github.com/matrix-org/matrix-react-sdk/compare/v0.12.3...v0.12.4-rc.1)
  