feat: dedicated login page for oauth2 logins

templates/layout_oauth2.html.twig

@@ -0,0 +1,14 @@
+{# Chrome for a login that is serving an external OAuth2 authorization.
+
+   Only the logo survives from the normal header: the account and logout links are dead ends in the
+   middle of an authorization, and the marketing column of the standard login page is noise when the
+   user did not come here to read about MyE&P. The footer's legal links stay -- this page collects
+   credentials, so dropping them would be the wrong direction.
+
+   The background is inherited unchanged, which is why the shared form partial's `text-white`
+   styling still works here. #}
+{% extends 'layout.html.twig' %}
+
+{% block header %}
+    {% include '_partials/_logo.html.twig' %}
+{% endblock %}