[FIX] crm: avoid constraint errors when merging partners

As for the _update_foreign_keys, the _update_reference_fields method may raise an unique constraint when merging two partners.
In such case, the new record is not relevant and can be removed.
[IMP] website_forum: performance improvements

- avoid to browse on every question/answer, only the 20 most recent ones (need to manually update the view to see the real number of q&a)
- do not render hidden tabs (leakage of information and useless rendering)
- add related fields to speed up vote search (need to be stored to be efficient)