message: Prepare changelog for v0.12.2
diff: +Changes in [0.12.2](https://github.com/matrix-org/matrix-react-sdk/releases/tag/v0.12.2) (2018-04-12)
  +=====================================================================================================
  +[Full Changelog](https://github.com/matrix-org/matrix-react-sdk/compare/v0.12.1...v0.12.2)
  +
  + * Null check stylesheet href
  + [\#1835](https://github.com/matrix-org/matrix-react-sdk/pull/1835)
  + * Remove the presence management labs feature
  +
  Changes in [0.12.1](https://github.com/matrix-org/matrix-react-sdk/releases/tag/v0.12.1) (2018-04-11)
  =====================================================================================================
  [Full Changelog](https://github.com/matrix-org/matrix-react-sdk/compare/v0.12.0...v0.12.1)
  + * Use correct js-sdk version
  Changes in [0.12.0](https://github.com/matrix-org/matrix-react-sdk/releases/tag/v0.12.0) (2018-04-11)
  =====================================================================================================
  