Properly migrate manuals

Redo implementation which duplicated manuals before.
Since these are datas, they should not be backed up, the xmlids need to be
renamed.

Depends on the following to be fully OpenUpgrade compatible:
* https://code.launchpad.net/~savoirfairelinux/openupgrade-server/rename_ir_model_data/+merge/231599
* https://code.launchpad.net/~savoirfairelinux/openupgrade-addons/document_page_ir_model_data/+merge/231615
stylistic remark: it is more efficient to write:

    if '.' not in old or '.' not in new:

as this does not involve paying the price for the splitting and will also stop at the first '.' in the general case.