Add rake task to add all users to a group.

What does this MR do?

Adds the rake task gitlab:import:all_users_to_a_group[group_name] to add all users to a single group.

Are there points in the code the reviewer needs to double check?

No

Why was this MR needed?

I needed to add all users to a group, but there was no rake task to do that. The closest was to add all users to all groups.

Screenshots (if relevant)

None

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Conforms to the code review guidelines
  • Has been reviewed by a UX Designer
  • Has been reviewed by a Frontend maintainer
  • Has been reviewed by a Backend maintainer
  • Has been reviewed by a Database specialist
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

None