Add robots meta tags on pages in robots.txt

Useful when app is at a non-root path and we're not handling top-level
/robots.txt requests.

app/templates/login.html

@@ -1,5 +1,8 @@
 {%- import "utils.html" as utils with context -%}
 {% extends "layout.html" %}
+{% block head %}
+<meta name="robots" content="noindex, nofollow">
+{% endblock %}
 {% block main_tag %} class="main-flex"{% endblock %}
 {% block content %}
 <div class="centered">