* adds blockstore as a requirement and an installed app, with
configurable bundle storage settings.
* adds waffle switch and setting to allow use of blockstore's python API
instead of REST API in live testservers and in production.
* adds database router which, when a `blockstore` DATABASE connection is
configured, allows the platform to use the blockstore service's
database instead of the default edxapp database.
* replaces blockstore_api exceptions and models with blockstore.app.api classes
* minor fixes to the blockstore_api to make the Blockstore REST API
return data packaged the same as the Blockstore Python API.
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