Release 0.12.0

• Fix spurious "unsaved changes" changeset in revision browser (#2259)
• Adds a new feature, file sources, which allows notebook authors to schedule
  the fetching of files from URLs. This feature is available in the notebook menu
  (click Menu > Manage Files) (#1985)
• Add multi stage docker builds for smaller docker images (#2156)
• Fixed a bug where script tags aren't being rendered properly that could lead
  to arbitrary code execution attacks (#2193)
• Fixes warnings caused by outdated version of Emotion (#2244)