Add Invite Members modal behind feature flag

What does this MR do?

Adds the UI component for Invite Members Modal as the first step in moving Invite Members form to a modal that can be triggered from multiple pages eventually.

• behind feature flag :invite_members_group_modal so the UI can be tested on staging (Feature flag rollout issue: https://gitlab.com/gitlab-org/gitlab/-/issues/247208)
• there is no User dropdown search yet, that will be added next iteration
• the trigger link in the side nav is behind the same feature flag for now, but the feature flag can be changed later to run separate experiments like https://gitlab.com/gitlab-org/gitlab/-/issues/224692 and to add further restrictions

Related to https://gitlab.com/gitlab-org/gitlab/-/issues/217922

Screenshots

On mobile:

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team