Invenio-pages

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- Major refactoring for Invenio 3.
- Adds versioning support for pages.

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- Removes calls to PluginManager consider_setuptools_entrypoints()
removed in PyTest 2.8.0.
- Adds missing `invenio_base` dependency.

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- Adds missing `invenio_upgrader` dependency and amends past upgrade
recipes following its separation into standalone package.
- Overrides default wtforms field for content column in order to
display it properly. (closes inveniosoftware/invenio3311)

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- Initial public release.