Web console: Page downloader, and fix JSON error resetting

[vogievetsky]

Added ability download pages of results from the new async APIs:

Also fixed the issue where a JSON error might cause the Next button to stay disabled if there is a json error while typing JSON

[adarshsanjeev]

Tried this out locally and it works great! Way better than manually making posts to get results.

I noticed something only while downloading the results, the format saved is as a json file, though according to the ResultFormat, ObjectLines is not valid json. (Firefox pointed this out too)
https://github.com/apache/druid/blob/master/sql/src/main/java/org/apache/druid/sql/http/ResultFormat.java#L96
Should the file extension be changed here?

I think part of the reason might be that the get results api is tagged as @Produces(MediaType.APPLICATION_JSON). I think I missed this while changing the default format. I'm not sure if the console takes this into account while deciding the file extension.

[adarshsanjeev]

Tried making the change to @Produces(MediaType.TEXT_PLAIN) locally to see if that fixes it, but the files downloaded are still .json.

I also noticed that the Edit context part of the UI has the plural as X keies in this PR (and not in the build of master), though I don't see any such change in the diff. Is this just me?

[vogievetsky]

Thank you for the testing and the feedback @adarshsanjeev I have addressed both issues in my commit. The extension is hard coded in the console right now and I changed it to jsonl which is more appropriate. Also I fixed the pluralizer