[MERGE] mail: provide fixes for gateway, composer and partner merge

Merge fixes coming notably from community reporting.

Containing
    [FIX] mail: handle wrong Final-Recipient header in incoming emails
    [FIX] mail: handle parsing of void body in incoming emails
    [FIX] mail: fix ACLs issue with mail composer in new mode
    [FW][IMP] test_mail: test models with type do not mess with attachmen…
    [FIX] mail: prevent useless write when pinning a channel
    [IMP] base: merge activities of merged partners

See sub commits for more details

Task-2641572

closes odoo/odoo#76159

Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>