Client should probe for valid server responses when started in online mode

[zenmonkeykstop]

In order for submissions to be downloaded and delete from the SecureDrop server, headers and capabilities need to be set correctly in the Apache2 config, as per freedomofpress/securedrop#4023 . If they're not, errors as per below will be observed in `sd-log:~/QubesIncomingLogs/sd-app/syslog.log:

Feb 18 16:52:37 localhost 2021-02-18 16:52:37,684 - securedrop_client.queue:179(process) ERROR: KeyError: 'Etag'
Feb 18 16:52:37 localhost 2021-02-18 16:52:37,687 - securedrop_client.logic:789(on_message_download_failure) ERROR: Could not emit message_download_failed: 'KeyError' object has no attribute 'uuid'

As a mitigation, the client could check to see if:

• expected headers are set in responses, including the correct ETag value for file transfers, which is a sha256 of the file IIRC
• expected HTTP methods are available
• (optional) server application code is up-to-date
• (optional) API version is compatible with that expected by the client

The specific errors above will no longer be encountered after an instance is migrated to v3 or Focal, as the Apache changes will be applied by either of these processes, so this is not immediately critical. It would be helpful to catch other potential problems, especially as part of a wider rollout or for workstation installs not supported by FPF.