fix: every icon of the templates comes from the local icon cache

django-bootstrap-icons serves icons from static/icon_cache and fetches a
missing one from the CDN while the page renders; a server without
internet access shows an error text instead of the icon.

- copy (Clone on the instance lists), envelope-open ("Lost OTP?" page)
  and save (VM settings, network page) were not cached; they are now,
  fetched with django-bootstrap-icons itself.
- The user and group lists asked for 'exclamation-triangle ' with a
  trailing space, an icon that does not exist; the warning icon never
  showed.

A test checks that every {% bs_icon %} in the templates has its cache
file; it fails on the previous files.

admin/templates/admin/group_list.html

@@ -17,7 +17,7 @@
     {% if not groups %}
         <div class="col-lg-12">
             <div class="alert alert-warning shadow-sm">
-                {% bs_icon 'exclamation-triangle '%} <strong>{% trans "Warning" %}:</strong> {% trans "You don't have any groups" %}
+                {% bs_icon 'exclamation-triangle' %} <strong>{% trans "Warning" %}:</strong> {% trans "You don't have any groups" %}
             </div>
         </div>
     {% else %}

admin/templates/admin/user_list.html

@@ -20,7 +20,7 @@
     {% if not users %}
         <div class="col-lg-12">
             <div class="alert alert-warning shadow-sm">
-                {% bs_icon 'exclamation-triangle '%} <strong>{% trans "Warning" %}:</strong> {% trans "You don't have any user" %}
+                {% bs_icon 'exclamation-triangle' %} <strong>{% trans "Warning" %}:</strong> {% trans "You don't have any user" %}
             </div>
         </div>
     {% else %}