Configurable list of things 'to be imported' that can be adjusted by the user

What does this MR do and why?

Related to #370734 (closed)

This MR adds support for advanced settings panel for importers

This is frontend counterpart of #373705 (closed)

Screenshots or screen recordings

Screencast_10_05_2022_02_49_03_PM

How to set up and validate locally

• Generate new GitHub token at https://github.com/settings/tokens
• open /import/github/new and use that token
• apply patch

index a836356e865d..a8523b84f258 100644
--- a/app/views/import/_githubish_status.html.haml
+++ b/app/views/import/_githubish_status.html.haml
@@ -5,7 +5,7 @@
 - paginatable = local_assigns.fetch(:paginatable, false)
 - default_namespace_path = (local_assigns[:default_namespace] || current_user.namespace).full_path
 - provider_title = Gitlab::ImportSources.title(provider)
-- optional_stages = local_assigns.fetch(:optional_stages, [])
+- optional_stages = local_assigns.fetch(:optional_stages, [{ name: 'feature1', label: 'Feature 1' }, { name: 'feature2', label: 'Feature 2', details: 'Feature 2 details'}])
 - header_title _("New project"), new_project_path
 - add_to_breadcrumbs s_('ProjectsNew|Import project'), new_project_path(anchor: 'import_project')

This is temporary solution while !98930 (merged) (backend counterpart) is not merged

• Observe panel and that on clicking Import / Import all buttons relevant parts of the request (optional_stages are included)

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.