Rework before_insert_items & before_update_items triggers

[zenovich]

1. Fix a bug in before_update_items causing new items with non-null url not matching any platform to have an unpredictable non-null platform_id. Update the related test.
2. Get rid of session variables in before_insert_items & before_update_items triggers because
   • they are not needed there,
   • use of session variables may lead to unpredictable results when we forget to initialize them with values (like it was in clause 1.).
3. Recalculate platform_id for all the items.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 100.00%. Comparing base (bcffaf5) to head (bbf3e2b).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## testhelpers_emptyDB_stability #1153 +/- ## =============================================================== Coverage 100.00% 100.00% =============================================================== Files 238 238 Lines 14542 14542 =============================================================== Hits 14542 14542 ```

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