Allow disabling editing of a M2M linked item

Summary I refer to discussion #16115. This is close to a deal breaker to me, because it causes so much confusion on the end user. Adding a linked item in a M2M field will enable editing the fields of the linked item in the relation dialog, falsy implying only this instance is edited. But if the user edits the head fields that related to the linked item, they will consequently influence the linked item where ever it has been used. Basic Example In the following example I have a link between a "nutrition_kit" and a "nutrition" table (M2M). In this example I pick a dataset from its M2M table from within the "nutrition_kit" form: <img width="945" alt="image" src=" Here the name field is not part of the M2M table, but instead part of the "nutrition" table that is linked in my M2M table. The user will be confused, might edit the name to further specify and thus editing the "nutrition" and not this r…