Not to create an account if already exist

[ysksn]

ActiveRecord::RecordNotUnique raised when running test because of the same Account created in db/seeds.rb.

ActiveRecord::RecordNotUnique:
  PG::UniqueViolation: ERROR:  duplicate key value violates unique constraint "accounts_pkey"
  DETAIL:  Key (id)=(-99) already exists.
  : INSERT INTO "accounts" ("id", "username", "private_key", "public_key", "created_at", "updated_at", "locked", "actor_type") VALUES ($1, $2, $3, $4, $5, $6, $7, $8) RETURNING "id"

Add instance actor (second take) #11321