Streamline identities CRUD + data model improvements

## DoD

- `edge-identities` APIs are deprecated.
- Identities management API and UI are the same for self-hosted and SaaS. That includes override counters and identity overrides in the feature flyout.
- Identity overrides are written to `environments_v2` table, after Core, not instead of it.
- Segments are written the same way to the `environments_v2` table as well.
- Trait updates are synced back to Core.

## Notes

Internally, segment and identity overrides could be stored simply as segment overrides. It's achievable after #5674 is done using the `$.identity.identifier` property in conjunction with the `IN` operator.