Migrate `app/views/projects/_gitlab_import_modal.html.haml` modal implementation to `GlModal`

The markup in app/views/projects/_gitlab_import_modal.html.haml replicates that of GlModal. It should be replaced with the implementation using GlModal. The modal display is handled by app/assets/javascripts/projects/project_new.js - line 103 that calls jquery's show method.