[FIX] mail: access link generaTION

Catch only the error related to the access of the linked record.
Previous was hidding error to access the ir.config_parameter and eventual other
errors from the orm.
Uses SUPERUSER_ID to access instance URL (e.g. portal has no access to it).
Fixes #4234 #4234
[FIX]web_gantt: value rendering with decimal separator other than dot
[MERGE] forward port of branch 7.0 up to 60a361c