Factor out the update button into its own small template

For code reuse, you know

templates/account.html

@@ -1,6 +1,7 @@
 {% extends 'layout.html' %}
 {% block main_content %}
 <h1>{{ account.name }}</h1>
+{% include 'update_button.html' %}
 <table class="table">
   <tr><td>Current balance</td><td>{{ account.balance }}</td></tr>
   <tr><td>Last updated</td><td>{{ account.last_updated }}</td></tr>