Adapt SecureDrop SDK to SecureDrop Proxy Behavior

[eloquence]

The SecureDrop Qubes OS proxy modifies requests with non-JSON responses (see https://github.com/freedomofpress/securedrop-proxy) - it uses qvm-move for those submissions. The SecureDrop SDK has to account for this behavior.

[kushaldas]

I think this part should be done by the proxy itself. For example, when the proxy gets a call to download a file, it will use the SDK API to download the file and then will use qvm-* command to move it to the right vm.
I will add an example below after getting back online.

[redshiftzero]

@kushaldas and I discussed this last week, and we decided that since securedrop-proxy has been completed and is ready to be packaged as discussed in freedomofpress/securedrop-workstation#107, we'll proceed with implementing the current plan for the MVP