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

## Screenshots

Before:

![Screen_Shot_2015-04-29_at_12.46.41_AM](https://gitlab.com/stanhu/gitlab-ce/uploads/16ea5a99cbace48cd2f2c238b5f73f4e/Screen_Shot_2015-04-29_at_12.46.41_AM.png)

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

![Screen_Shot_2015-04-30_at_1.34.38_AM](https://gitlab.com/gitlab-org/gitlab-ce/uploads/43c612cdcbab181713e5764e2f38a04e/Screen_Shot_2015-04-30_at_1.34.38_AM.png)

After with 1 valid and 1 incompatible project:

![Screen_Shot_2015-04-30_at_1.37.26_AM](https://gitlab.com/gitlab-org/gitlab-ce/uploads/b5bcbd304206a996932e5208ef54a071/Screen_Shot_2015-04-30_at_1.37.26_AM.png)

See merge request !586