Select field/column content from SCIO-DB for XLSX output

[mjherzog]

In addition to selecting codebase subsets from a Project in SCIO-DB (see https://github.com/nexB/scancode.io/issues/48), we often want to extract a specific subset of columns for a particular type of analysis in Excel/Calc. For example, you might want just the Copyright and License fields/columns for Files and Packages without the license match_rule data and without the other Package data. It would be ideal to have some way to select the fields/columns you want to extract from a list with check boxes or similar UI. We will, of course, want to combine this with the capability to select Codebase rows in one UI. That combination would be essentially be the first version of an SCIO Reports module.