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
An even more efficient way would be to split it before the if to avoid spliting twice.