fixes#4607
- moves file checks from db and upload API endpoints to api utils
- adds code to accept and then extract a zip and pull out a JSON file
- zip handling requires a lot of dependencies - this needs a good refactor
addresses #2852
- Moves storage modules to use prototypes and to create prototypes
that inherit from the base storage ctor.
- Makes storage/base conform to an all Promise interface.
closes#3252
- added `/ghost/api/v0.1/uploads/` endpoint
- removed upload method from `controller/admin.js`
- moved removal of temporary files from storage to endpoint (needed to
account for failed uploads)
- changed and moved tests
- Oversight: I think that we use `.otherwise()` and `.catch()` a bit
too extensive and mask the real error objects. We probably need an
error handling strategy at some point in the future.
