Ftw.upgrade

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- Add method to remove a previously uninstalled portlet manager from the
persistent registry.
[deiferni]

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- Fix support for GS import by tarball upload. [jone]

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- Add upgrade step method ``ensure_profile_installed(profileid)``. [jone]

- Add upgrade step method ``is_profile_installed(profileid)``. [jone]

- Add upgrade step method ``is_product_installed(product_name)``. [jone]

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- Migration: fix error when file obj is an empty string. [jone]

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- Migration: do not migrate empty blobs. [jone]

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- Migration: support all types of standard relation fields. [jone]