Use the static temlate tag, not STATIC_URL

STATIC_URL has not been advocated by Django for many years now. The
static template tag deals properly with multiple static providers and
those which are not just local files.

Signed-off-by: Mattia Rizzolo <mattia@debian.org>

signon/templates/signon/audit_identities.html

+{% load static %}
 {% load i18n %}
 
 {% for identity in identities %}
@@ -6,7 +7,7 @@
     {% with identity.get_provider as provider %}
     {% if identity.profile %}<a href="{{identity.profile}}">{% endif %}
       {% if provider.icon %}
-      <img style="vertical-align: text-top; height: 1em" class="mr-2" src="{{STATIC_URL}}{{provider.icon}}"></img>
+      <img style="vertical-align: text-top; height: 1em" class="mr-2" src="{% static provider.icon %}"></img>
       {% else %}
       <span class="mr-2 fa fa-sign-in"></span>
       {% endif %}

signon/templates/signon/login.html

 {% extends base_template %}
+{% load static %}
 {% load i18n %}
 
 {% block content %}
@@ -25,7 +26,7 @@
     <li class="row">
       <div class="btn btn-info btn-lg disabled col-sm-6">
         {% if provider.icon %}
-        <img style="vertical-align: text-top; height: 1em" class="mr-2" src="{{STATIC_URL}}{{provider.icon}}"></img>
+        <img style="vertical-align: text-top; height: 1em" class="mr-2" src="{% static provider.icon %}"></img>
         {% else %}
         <span class="mr-2 fa fa-sign-in"></span>
         {% endif %}
@@ -89,7 +90,7 @@
       <li class="row mx-0 my-2">
         <a class="btn btn-primary btn-lg col-md-6" href="{{provider.get_authorization_url}}" role="button">
         {% if provider.icon %}
-        <img style="vertical-align: text-top; height: 1em" class="mr-2" src="{{STATIC_URL}}{{provider.icon}}"></img>
+        <img style="vertical-align: text-top; height: 1em" class="mr-2" src="{% static provider.icon %}"></img>
         {% else %}
         <span class="mr-2 fa fa-sign-in"></span>
         {% endif %}