Add feature flag check to `New organization` option in nav dropdown

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 !148347 (merged) and !147930 (merged) we introduced a allow_organization_creation feature flag to control who can create organizations for the first iteration. We missed adding this feature flag to the New organization dropdown item in the main sidebar. This MR adds the feature flag check there.

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Before	After

How to set up and validate locally

• Enable ui_for_organizations feature flag
  • echo "Feature.enable(:ui_for_organizations)" | gdk rails c
• Open the dropdown in the sidebar navigation. The New organization button should not be there
• Enable allow_organization_creation feature flag
  • echo "Feature.enable(:allow_organization_creation)" | gdk rails c
• Open the dropdown in the sidebar navigation. The New organization button should be there