Improve "import in progress" screen

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Right now, if you import a project and it takes more than a few seconds, the page will continue auto-refreshing itself. This looks sloppy as the page will "stutter" every few seconds as the page re-renders. We should use javascript polling on the frontend to make this more seamless.

We also ought to replace the old loading spinner with the new design from gitlab-ui.

We could also take a page from github's design and place a friendly SVG illustration on this page as they do when forking/importing a repository.