Move organization_switching to org_creation release stage

What does this MR do and why?

Moves the organization_switching feature flag to an organization flag. The organization_switching feature flag is a confusing name. At one point it controlled showing the organization switcher and organization creation. Showing the organization switcher has been moved behind the org_switcher release stage so now the organization_switching feature flag only controls organization creation. This is why this MR uses the name org_creation.

How to set up and validate locally

1. Go to /rails/features and enable the org_stage_experimental feature flag
2. You should be see a New organization option in the + button in the top bar
3. You should be able to create an organization using this form
4. Go to /rails/features and disable the org_stage_experimental feature flag
5. You should no longer be able to create an organization

MR acceptance checklist

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