[UX] Support importing D7 views exports (at least in the 1.x cycle)

I am opening this as both a feature request, and as documentation, because it's been now at least twice that I saw this being asked by newcomers on Gitter.

The current way to be importing D7 views post-installation, is to be using the contrib module https://github.com/backdrop-contrib/default_views_config to convert exported D7 views into config, and then to import those into Backdrop via /admin/config/development/configuration/single/import.

With all the expected D7 sites upgrading to Backdrop over the next couple of years, we should make it easier to do that in core (we could remove that in Backdrop 2.0).

At the very least, we should make sure that the process of using the contrib module is documented in a place where people would expect to find it. Perhaps even consider a help message in Views, pointing to the contrib solution + online documentation.