Support `FormData` objects in `ReactServerAgent.send(...)`

[roblg]

We currently don't support sending FormData with .send(...) on RSA requests, even though superagent does. We should support it, at least when making calls from the client-side. (It's possible that it would be reasonable to make calls from server-side too, but we'd have to figure out how to serialize the data, and how to do equality checking against the data cache on the frontend.)