Add subtitles to articles

Fix #152

templates/posts/details.html.tera

@@ -11,6 +11,7 @@
 
 {% block content %}
     <h1 class="article">{{ article.post.title }}</h1>
+    <h2 class="article">{{ article.post.subtitle }}</h2>
     <p class="article-info">
         <span class="author">{{ "Written by {{ link_1 }}{{ url }}{{ link_2 }}{{ name | escape }}{{ link_3 }}" | _(
             link_1='<a href="/@/',

templates/posts/new.html.tera

@@ -9,6 +9,7 @@
 <h1>{{ "Create a post" | _ }}</h1>
 <form class="new-post" method="post">
     {{ macros::input(name="title", label="Title", errors=errors, form=form, props="required") }}
+    {{ macros::input(name="subtitle", label="Subtitle", errors=errors, form=form, optional=true) }}
 
     {% if errors is defined and errors.content %}
         {% for err in errors.content %}