Ftw.upgrade

----------------

- Add installed upgrades to transaction note. Closes 7
[jone]

- Add ``migrate_class`` helper with _p_changed implementation supporting BTreeFolder2Base containers.
[jone]

- Remove ``purge_resource_registries()`` helper because it does not behave as expected.
[jone]

- Set min-height of upgrade output frame to 500px.
[jone]

- Print exceptions to browser log stream.
[jone]

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- Fix the upgrade registration problem (using a classmethod does not work
since registration fails).
[jone]

- Let manage-upgrade be usable without actually installing the GS profile.
[maethu]

------------------

- First implementation.
[jone]