Text export from JobQuery page

[lars-t-hansen]

@ashenr @PubuduSaneth

After discussions a feature request: the JobQuery page should be able to export its results as a text table. What we're envisioning (let's discuss) is that the JobQuery page has a button or link thing that pops up a new tab. The tab has some selectors: for the fields of interest (and for those that have multiple units, the unit of interest), and for the format (typically the ones supported by the backend: CSV, CSV with field names, AWK (space-separated), JSON, with and without a header line. The default should be CSV with a header as this is most useful for most people. All the fields that can be printed should be selectable. Then there's a button to press to generate the report from the query, this can be processed on the back-end. The text should then be presented in a textarea that is easily copied.

On the backend this is just a jobs query, there is nothing new that needs to be implemented I think. The logic is much the same as in the JobQuery page but the output format is not restricted to JSON.