Implement register endpoint for m.login.dummy login type

Add device table

lib/matrix_server/device.ex

@@ -0,0 +1,32 @@
+defmodule MatrixServer.Device do
+  use Ecto.Schema
+  import Ecto.Changeset
+  alias MatrixServer.{Account, Device}
+
+  @primary_key false
+  schema "devices" do
+    field :device_id, :string, primary_key: true
+    field :access_token, :string
+    field :display_name, :string
+
+    belongs_to :account, Account,
+      foreign_key: :localpart,
+      references: :localpart,
+      type: :string,
+      primary_key: true
+  end
+
+  def changeset(device, params \\ %{}) do
+    device
+    |> cast(params, [:localpart, :device_id, :access_token, :display_name])
+    |> validate_required([:localpart, :device_id])
+    |> unique_constraint([:localpart, :device_id], name: :devices_pkey)
+  end
+
+  def generate_access_token(repo, %{device: %Device{localpart: localpart, device_id: device_id} = device}) do
+    access_token = Phoenix.Token.encrypt(MatrixServerWeb.Endpoint, "access_token", {localpart, device_id})
+    device
+    |> change(%{access_token: access_token})
+    |> repo.update()
+  end
+end