Upgrade react-mosaic-component and stop using the fork

[jtbandes]

react-mosaic was forked for reasons described at https://gist.github.com/troygibb/9be30ae863ea5da4ebdbf58f1eda8f49. We are now able to move off the fork with these adaptations:

• Instead of reading mosaicId from the root mosaic component, just pick a uuid, and pass that in to all mosaics (thanks to Configurable mosaic ID nomcopter/react-mosaic#143).
• Instead of MosaicDumbWindow and removeRootDropTarget, use a little bit of hacky CSS to add display: none to the unsavory drop targets.
• Instead of threading tabId everywhere through the mosaic source code, add tabId using a feature of react-dnd where drop targets can read/overwrite drag context from nested drop targets.