Hide organization switcher for users with one organization

What does this MR do and why?

Related to #567524

Organization switching is and will only be available to GitLab team members for the foreseeable future for testing purposes. To avoid confusion for GitLab team members that do not have multiple organizations and to un-clutter the navigation we are going to hide the organization switcher for users with only one organization.

References

#567524

Screenshots or screen recordings

New UI

One organization	Multiple organizations

Old UI

One organization	Multiple organizations

How to set up and validate locally

1. Go to /rails/features and enable the following feature flags
   • ui_for_organizations
   • organization_switching
   • organization_scoped_paths
2. Go to /-/organizations/new and create an organization
3. You should see the switcher now that you have multiple organizations
4. Go to /admin/users and impersonate a user that only has one organization (they all should unless you explicitly created an organization with another user)
5. You should not see the switcher

MR acceptance checklist

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