Plonemeeting.restapi

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- Fixed `base.serialize_annexes`, make sure we get no annex if the given filters gives no uids.
Passing no uids to get_categorized_elements means `Do not filter on uids`.
[gbastien]

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- Restored `Products.PloneMeeting 4.1.x/4.2.x` backward compatibility.
[gbastien]

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- Only display the `Unknown data` warning when creating an element if returning
full obj serialization after creation.
[gbastien]
- Fixed creation of meeting with annexes.
[gbastien]
- Make the annex serializer include `file` in base data.
[gbastien]
- Fixed `clean_html=False` when creating DX content, `clean_html` was always applied.
[gbastien]

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- Added upgrade step to 2000 that will re-apply the `rolemap` step so we are
sure old installations are restricting the service to role `Member`.
[gbastien]

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- Added HTML clean (enabled by default) when adding an element (AT or DX).
[gbastien]
- Added `extra_include=annexes` available on item and meeting.
[gbastien]

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- Make sure every `extra_include` are correctly defined in
`_available_extra_includes`. Now if not defined there, it will be ignored.
[gbastien]