Add a "Download as CSV" to DefaultController

[hellopablo]

Add a header button to download the current result set (either the page, or the entire set) as a CSV.

Things to consider:

• Large datasets
  • Perhaps offload to DataExport?
• What to include in the D/L? Just the columns visible on screen, or the whole object.
  • If whole object, what about expanded fields and non-scalar fields.

[hellopablo]

@gsdd This would mean in many cases clients would not need a Data Export source built, and they could just download straight from admin - which would be cool - but do you have thoughts on how we handle the above?