Enhancement suggestion - Provide option to delete storage on repo delete, or add an explanation

[iddoavn]

Today, when deleting a repo, the underlying storage is kept.

If, in the future, someone wants to recreate a repository using the same storage namespace, they will get an error message.
Furthermore, it leaves wasted storage.

It might be worth:

1. Telling the user they should delete the data, or, alternatively,
2. Allowing an option to delete the data on the deletion of the branch (which is risky for many other reasons :) ), or,
3. delete only the files that will prevent from a new repository being created on the same namespace.

[github-actions]

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[talSofer]

Discussed with @ozkatz and we decided to add an explanation with instructions for how to delete the storage namespace.