Merge pull request 'Add grouped notifications API' (#7910) from issue-7905-grouped-notifications into develop

Reviewed-on: https://git.pleroma.social/pleroma/pleroma/pulls/7910
Reviewed-by: Phantasm <phnt@noreply.git.pleroma.social>

priv/repo/migrations/20200602125217_add_group_key_to_notifications.exs

@@ -0,0 +1,15 @@
+defmodule Pleroma.Repo.Migrations.AddGroupKeyToNotifications do
+  use Ecto.Migration
+
+  def change do
+    alter table(:notifications) do
+      add(:group_key, :string)
+    end
+
+    create_if_not_exists(
+      index(:notifications, [:user_id, :group_key, "id desc nulls last"],
+        where: "group_key IS NOT NULL"
+      )
+    )
+  end
+end