Introduce a feature flag for using new Account structure

Proposal

To deliver features more efficiently and safely, let's introduce a feature flag that can be used to gate the new logic for using Accounts and AccountMemberships. The feature flag could be named something like account_data_structure but open to other suggestions. When the flag is enabled, the new data structure would be used, otherwise continue to use the old relationships. Even when the flag is disabled though, we could start populating Accounts and Account Memberships to get into a "compatibility mode" where we could easily switch the flag on and off as needed.

TODOs:

Edited by Vladlena Shumilo