Silva-core-upgrade

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* ...

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* Convert Silva File and Silva Image to Silva Ghost Asset if they are
located inside a valid Silva Ghost Folder and the original files
exists.

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* Support upgrade of ExtFile based files to blob storage.

* Add upgrade for silva.core.editor to 3.0.3.

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* Improve upgrade commands by adding the support for partial upgrade
and upgrade selection using tags.

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* Rewrite upgrader registry to optimize upgrade speed.

* Add upgraders to Silva 3.0c1.

* Fix upgrade from Silva 2.1 to 3.0.

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* Add upgraders to Silva 3.0b1.

* Remove old upgraders before 2.1.

* Base the command script on ``silva.system.utils``.

* Extension upgraders (SilvaFind, SilvaNews) moved to theirs
respective extensions.