Switch existing command to standard approach

This is the suggested approach in the documentation [1] and using
it makes it clearer what's going on and to add other commands with
arguments, which we'll do in the next commit.

[1]: https://flask.palletsprojects.com/en/2.0.x/cli/#custom-commands

app/commands.py

@@ -1,6 +1,10 @@
+import click
 from flask import current_app
+from flask.cli import with_appcontext
 
 
+@click.command('list-routes')
+@with_appcontext
 def list_routes():
     """List URLs of all application routes."""
     for rule in sorted(current_app.url_map.iter_rules(), key=lambda r: r.rule):
@@ -8,4 +12,4 @@ def list_routes():
 
 
 def setup_commands(application):
-    application.cli.command('list-routes')(list_routes)
+    application.cli.add_command(list_routes)