[12.0][MIG] product_unit_manager_group + rename

[clementmbr]

Basic module migration and :

• rename module's name product_unit_manager_group into uom_manager_group in order to follow the new separation between product and uom modules in Odoo 12.0
• add readme
• light refactor adding a group category "Units of Measure" with two levels :
  • User : UoM fields are displayed but not Uom menus (Users are not allowed to create or modify UoM)
  • Manager : display both fields and UoM menus
  • nothing : neither UoM fields nor menus are displayed

The rest of the module philosophy is maintained. All the Sales Manager, Stock Manager, MRP Manager have no more rights to create or modify UoMs. Only "UoM Managers" can.

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TODO : migration script to change module name during migration

cc @sebastienbeau