Group push rules: populate `group_push_rules` with group records from `push_rules`

Problem

group_push_rules contains records created after Group push rules: execute write operations for ... (#498934 - closed). However, it's missing old historical records from push_rules.

Proposal

Add background migration to copy old group push rules records into group_push_rules table. We should keep the id of tables identical for same records to have a way to verify if they are in sync.