Create project: Add second step to an import project

What does this MR do and why?

Add second step to an import project form

⚠️ ⚠️ ⚠️ These changes are behind the new_project_creation_form feature flag. They won't be available to users until all the work is done. See the plan here #514700 (closed) ⚠️ ⚠️ ⚠️

Screenshots or screen recordings

How to set up and validate locally

1. In rails console enable the experiment fully

   Feature.enable(:new_project_creation_form)

2. Go to http://gdk.test:3000/projects/new

3. Choose [Import project]

4. See a list of available imports

5. You can click through them

To see how availability works, go to settings and switch on/off some import sources.

Links

Project creation plans: #514700 (closed)

Milestone planning issue: #499697 (closed)