Add support for Group-level project templates

What does this MR do?

It adds support to specify a Subgroup containing project templates through the settings of a top level Group.

When creating a project based on a Group's project template, the new project should be created under the same hierarchy of that project template. It means you can't use a project template from Group A and create the new project in Group B, you can only use Group A or any subgroup of Group A as the namespace of the new project.

What are the relevant issue numbers?

For #6861 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• EE specific content should be in the top level /ee folder
• For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?

Closes #6861 (closed)