Check `allow_organization_creation` feature flag on a few more views

What does this MR do and why?

Related to #452518 (closed)

The grouptenant scale team is starting to build out an Organization MVC. For more information about what an "Organization" will be see https://docs.gitlab.com/ee/architecture/blueprints/organization/

In !147930 (merged) we introduced a new allow_organization_creation to control who can create an organization for the MVC. There were a few places that needed to be updated for full parity. This MR updates following logic when feature flag is disabled:

1. Hide New organization button in empty state
2. 404 /-/organizations/new
3. Hide New organization button in admin area (/admin/organizations)

MR acceptance checklist

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Screenshots or screen recordings

With allow_organization_creation feature flag disabled

Your work -> Organizations

Before	After

Admin area -> Organizations

Before	After

How to set up and validate locally

1. Enable ui_for_organizations feature flag
   • echo "Feature.enable(:ui_for_organizations)" | gdk rails c
2. Change app/assets/javascripts/organizations/shared/components/organizations_view.vue#L35 to return []; to force empty state
3. Navigate to /admin/organizations
   • New organization buttons should not be shown
4. Navigate to /-/organizations
   • New organization buttons should not be shown
5. Navigate to /-/organizations/new
   • You should get a 404
6. Enable allow_organization_creation feature flag and go through steps 3 - 5 again. It should now be possible to create an organization.
   • echo "Feature.enable(:allow_organization_creation)" | gdk rails c