Implement register endpoint for m.login.dummy login type

Add device table

priv/repo/migrations/20210623145023_add_devices_table.exs

@@ -0,0 +1,18 @@
+defmodule MatrixServer.Repo.Migrations.AddDevicesTable do
+  use Ecto.Migration
+
+  def change do
+    create table(:devices, primary_key: false) do
+      add :device_id, :string, primary_key: true, null: false
+      add :access_token, :string
+      add :display_name, :string
+
+      add :localpart,
+          references(:accounts, column: :localpart, on_delete: :delete_all, type: :string),
+          primary_key: true,
+          null: false
+    end
+
+    create index(:devices, [:localpart])
+  end
+end