[MERGE][IMP] portal, various: add access tokens in document access button for customer notification emails

Previously to this merge portal customers receiving a notification email
on a shared document could not directly reach the document on the portal
as the access token was not given to them. The only way to give access
to portal was done using the specific email implemented for sale orders
or invoices. This behavior is now extended to notification emails.

This merge adds the token support for customers through inheritance in
portal mixin. It also fixes inherit order so that the call chain
effectively goes through portal before calling the mail.thread base
implementation.

This merge is related to task 40064. #Closes #21426.

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