add user posts view

app/templates/users/sign_up.html

@@ -1,3 +1,4 @@
+{% from 'common/macros.html' import infobox with context %}
 {% from 'common/macros.html' import subheader %}
 {%- extends 'base.html' -%}
 {%- block title -%}sign up{%- endblock -%}
@@ -7,9 +8,7 @@ Please read the rules etc. stub
 {%- endset -%}
 {{ subheader('Sign up', welcome)}}
 {%- if request.args.get('error') -%}
-<div class="infobox plank critical">
-  {{request.args.get('error')}}
-</div>
+  {{infobox(request.args.error, InfoboxKind.ERROR)}}
 {%- endif -%}
 <form class="plank primary-bg full-width" method="POST">
   <label for="username">Username</label>