Goal: 

• Currently we don't have an easy way for the UI to check if an artifact is already uploaded, and instead we upload it again.
• So if something happens (session times out), the user has to re-upload everything again
• The UI is calculating the checksums, so it would be easy for it to send a request with a bunch of checksums and get back a list that are already present 

Acceptance Criteria:

something like:

• POST  /repositories/uploads/search {  sha256: ["abcde", "efgh"]  }

returning: