Show purchased storage UI only in dotcom

Summary

The purchased storage new UI which is behind additional_repo_storage_by_namespace feature flag is visible on Self managed instances as well. Storage purchase workflow is for dotcom alone. This issue is to figure out how the UI can achieve this.

One way to possibly achieve this is to check for automatic_purchased_storage_allocation Application setting

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Edit 2023-01-27

We can implement this by adding a check on the application setting should_check_namespace_plan, since checking .com? is discouraged: &7374 (closed).

MR implementing this issue: !109806 (merged)