Move image upload to API

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.