Add alias creation endpoint

lib/matrix_server_web/controllers/aliases_controller.ex

@@ -0,0 +1,22 @@
+defmodule MatrixServerWeb.AliasesController do
+  use MatrixServerWeb, :controller
+
+  import MatrixServerWeb.Plug.Error
+
+  alias MatrixServer.Alias
+
+  def create(conn, %{"alias" => alias, "room_id" => room_id}) do
+    case Alias.create(alias, room_id) do
+      {:ok, _} ->
+        conn
+        |> put_status(200)
+        |> json(%{})
+
+      {:error, cs} ->
+        put_error(conn, Alias.get_error(cs))
+    end
+  end
+
+  # TODO: create error view for this?
+  def create(conn, _), do: put_error(conn, :bad_json)
+end