Add explicit user selection for asTale parameter

[craig-willis]

With WT now enabled on the Dataverse demo site I'm returning to the discussion of how to handle the asTale flag during Analyze in WT. Today, the Dataverse integration defaults to asTale=true, because it makes sense when demonstrating repo2docker-compatible datasets. But this isn't always the case.

Per the 10/24/2019 UI/UX meeting we have mockups demonstrating explicit user selection of how to handle input data during Analyze in WT.

( https://whole-tale.github.io/wholetale-css-mockup/src/browse.html , select "Create Tale" > "Simulate Analyze in WT for tale-like")

[craig-willis]

For now link to http://docs.wholetale.org/en/stable/users_guide/compose.html (new tab/window). This may change after whole-tale/wt-design-docs#116