add first-in-first-out queue for image prompts, and add pagination

priv/repo/migrations/20220830171344_add_steps_and_guidance_scale_to_prompt_request.exs

@@ -0,0 +1,10 @@
+defmodule Diffuser.Repo.Migrations.AddStepsAndGuidanceScaleToPromptRequest do
+  use Ecto.Migration
+
+  def change do
+    alter table(:prompt_requests) do
+      add :steps, :integer
+      add :guidance_scale, :float
+    end
+  end
+end