* Add group model
```sql
BEGIN;
--
-- Create model Group
--
CREATE TABLE "posthog_group" ("id" serial NOT NULL PRIMARY KEY, "group_key" varchar(400) NOT NULL, "group_type_index" integer NOT NULL, "group_properties" jsonb NOT NULL, "created_at" timestamp with time zone NOT NULL, "properties_last_updated_at" jsonb NOT NULL, "properties_last_operation" jsonb NOT NULL, "version" bigint NOT NULL, "team_id" integer NOT NULL);
--
-- Create constraint unique team_id/group_key/group_type_index combo on model group
--
ALTER TABLE "posthog_group" ADD CONSTRAINT "unique team_id/group_key/group_type_index combo" UNIQUE ("team_id", "group_key", "group_type_index");
ALTER TABLE "posthog_group" ADD CONSTRAINT "posthog_group_team_id_b3aed896_fk_posthog_team_id" FOREIGN KEY ("team_id") REFERENCES "posthog_team" ("id") DEFERRABLE INITIALLY DEFERRED;
CREATE INDEX "posthog_group_team_id_b3aed896" ON "posthog_group" ("team_id");
COMMIT;
```
* Remove a dead import
* Improve typing for groups
* Make groups updating more generic, avoid mutation
This simplifies using the same logic for groups
Note there's a behavioral change: We don't produce a new kafka message
if nothing has been updated anymore.
* Rename a function
* WIP: Handle group property updates
... by storing them in postgres
Uses identical pattern to person property updates, except we handle
first-seen case within updates as well.
* Get rid of boolean option
* WIP continued
* fetchGroup() and upsertGroup()
* Test more edge cases
* Add tests for upsertGroup() in properties-updater
* Rename to PropertyUpdateOperation
* Followup
* Solve typing issues
* Cleanup nits
* Add test showing desired behavior during insert race
* Handle race conditions gracefully
