Refactor ServerClient to handle MultipartRequests

[jwr1]

• Create a new method in ServerClient, postMultipart which contains properties for fields (map of Strings) and files (map of XFiles).
• Remove unnecessary headers argument from existing methods. It was only used to specify a JSON content type, which is done automatically when body is provided.
• Stop using ServerClient's HTTP client for external requests, as that passes them authentication headers, and would essentially give them full access to the signed-in account if they knew what to do with it. Let's just hope neither image hosting service is malicious.