Enable compression option to configure whether IDS downloads are compressed

[louise-davies]

Currently, the zipType option specified in topcat.json downloadTransportType is effectively ignored. We received a user feedback asking us why we weren't compressing HTTPS downloads, and Chris explained that previously this wasn't enabled due to load on the IDS machine, but perhaps this should be re-evaluated. At the very least, since we have a config option we should allow this option to actually configure the calls to the IDS.

Note: this will likely involve adding a zipType property to the Download entity (and hence a database schema change) to enable the StatusCheck call to prepareData to use the config option.

[louise-davies]

Due to current Diamond priorities this feature won't be able to be tested until at least the Payara 5 upgrade and CentOS 7 migrations are completed, and so this can wait until those two tasks are closer to completion