Convert data download to POST to avoid long URLs

[rod-glover]

Update: This may not be necessary if we are prepared to ditch IE/Edge support (:tada:). All other browsers support URLs of at least 8k chars, and servers do too. In that case we could configure to allow long URLs and not go to all this trouble. See #143 for more info.

This must be coupled with a more complicated arrangement in the server. One method would be to return a URL in the POST response that the client (this app) GETs. That URL should be formed from a deterministic (but short) hash of the parameters passed in the POST. The client can trigger a download programmatically once it receives the response from the POST.

[rod-glover]

I have encountered a couple of suggestions online that the download from a post can somehow be managed directly, but I am not convinced of this ... and, FWIW, you would lose the caching of the actual data download, which you keep with the deterministic hash-based return URL.