Show incompatible projects in Google Code import status

Using Google Code import with a JSON file that contained only one Subversion project led to confusion over whether the system was working. Display the list of valid projects if there are any, and show a list of incompatible projects. Provide tips on how to retain issue data after conversion.

Closes #1531 (closed)

Screenshots

Before:

After with no projects available (notice the button is hidden):

After with 1 valid and 1 incompatible project: