Add membership radio buttons to invite members modal

What does this MR do and why?

Add membership radio buttons to invite members modal

Adds feature-flag-gated radio buttons allowing owners to invite members to the root group from a project or subgroup.

Changelog: added

References

Screenshots or screen recordings

Top Level Group

Sub Group

Project in Group

How to set up and validate locally

1. Enable feature flag invite_to_root_group (gdk rails c -> Feature.enable(:invite_to_root_group))

From a top-level group

1. Navigate to a top-level group
2. Go to Manage > Members
3. Press the Invite members button (should not see the radio options)

From a sub group

1. Navigate to a group under another group
2. Go to Manage > Members
3. Press the Invite members button (should see the radio options)
4. Set to either the sub group, or top level group

From a project

1. Navigate to a project under the group
2. Go to Manage > Members
3. Press the Invite members button (should see the radio options)
4. Set to either the project, or top level group

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Related to #590429