Fixes for CSV support.

* Fix incorrect reference to `calculateVarMinMax` in Variable.js.
* Don't set the Scale to NaN when a column has a mix of numbers and strings.
Replace Data and Map icons with text.
Add a "Legend" button when the topmost layer has a legend.

Clicking the button opens the legend URL in a separate tab.
Merge remote-tracking branch 'origin/csvFixes' into refactor
Merge remote-tracking branch 'origin/legend' into refactor

Conflicts:
	src/viewer/GeoDataBrowserViewModel.js