Fix namespace_id being lost for github import

What does this MR do and why?

This MR ensures we're passing namespace_id to github importer when importing project via subgroup

Related issues:

• GitHub Importer - Default to the current group ... (#353936 - closed)
• Importers - Default to the current group when i... (#359130 - closed)

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Create new access token on github https://github.com/settings/tokens/new
2. Open any group on your installation (for example jashkenas for default GDK installation)
3. Click New project -> Import project
4. Verify that Github button includes namespace_id (for example /import/github/new?namespace_id=29)

(next steps are optional and verify actual functionality)

1. Click GitHub link and use your access token
2. Verify that namespace_id is present in URL
3. Verify that default group in dropdown is selected to group you've used to initiate project creation

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

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