Add migration and schema for accounts

Add account name availability endpoint

lib/matrix_server_web/controllers/account_controller.ex

@@ -0,0 +1,28 @@
+defmodule MatrixServerWeb.AccountController do
+  use MatrixServerWeb, :controller
+  alias MatrixServer.Account
+
+  def register(conn, _params) do
+    conn
+  end
+
+  def available(conn, params) do
+    localpart = Map.get(params, "username", "")
+
+    {status, data} =
+      case Account.available?(localpart) do
+        :ok ->
+          {200, %{available: true}}
+
+        {:error, :user_in_use} ->
+          {400, %{errcode: "M_USER_IN_USE", error: "Desired user ID is already taken."}}
+
+        {:error, :invalid_username} ->
+          {400, %{errocode: "M_INVALID_USERNAME", error: "Desired user ID is invalid."}}
+      end
+
+    conn
+    |> put_status(status)
+    |> json(data)
+  end
+end