[IMP] fields: date, datetime: added MONTHS variable holding list of (month_number, month_name)
[DOC] Added changelog for this branch
[MERGE] [IMP] fields: date, datetime: added MONTHS variable holding list of (month_number, month_name) + [DOC] updated changelog

This allows to remove the definition of MONTHS that appear in several addons, notably in reporting. Addons branches will come that use this new attribute
[FIX]ir_attachement: if res_id = 0, allow to read if allowed to read model