RunService.addRunFromData() returns text/plain

[johnaohara]

          I had to explicitly state the api produces `text/plain` to fix this issue in the UI:

even if the upload was working

Originally posted by @lampajr in #2243 (comment)

atm it just returns an array of ID's for the accepted Runs to process.
we should return a properly formatted JSON object, that contains the URI for each run, e.g https://restfulapi.net/http-status-202-accepted/