Dispute Document API

[kyungmin]

Since the dispute portal launched, there have been lot of requests for an dispute document API, to avoid manual uploads:

https://github.com/balanced/balanced-api-private/issues/14
https://github.com/balanced/balanced-api-private/issues/24
#290 (comment)

[remear]

#571