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.

static/icon_cache/copy_None_None_None.svg

@@ -0,0 +1,8 @@
+<?xml version="1.0" ?>
+<svg xmlns="http://www.w3.org/2000/svg" width="16" height="16" fill="currentColor" class="bi bi-copy" viewBox="0 0 16 16">
+	
+  
+	<path fill-rule="evenodd" d="M4 2a2 2 0 0 1 2-2h8a2 2 0 0 1 2 2v8a2 2 0 0 1-2 2H6a2 2 0 0 1-2-2zm2-1a1 1 0 0 0-1 1v8a1 1 0 0 0 1 1h8a1 1 0 0 0 1-1V2a1 1 0 0 0-1-1zM2 5a1 1 0 0 0-1 1v8a1 1 0 0 0 1 1h8a1 1 0 0 0 1-1v-1h1v1a2 2 0 0 1-2 2H2a2 2 0 0 1-2-2V6a2 2 0 0 1 2-2h1v1z"/>
+	
+
+</svg>

static/icon_cache/envelope-open_None_None_None.svg

@@ -0,0 +1,8 @@
+<?xml version="1.0" ?>
+<svg xmlns="http://www.w3.org/2000/svg" width="16" height="16" fill="currentColor" class="bi bi-envelope-open" viewBox="0 0 16 16">
+	
+  
+	<path d="M8.47 1.318a1 1 0 0 0-.94 0l-6 3.2A1 1 0 0 0 1 5.4v.817l5.75 3.45L8 8.917l1.25.75L15 6.217V5.4a1 1 0 0 0-.53-.882l-6-3.2ZM15 7.383l-4.778 2.867L15 13.117zm-.035 6.88L8 10.082l-6.965 4.18A1 1 0 0 0 2 15h12a1 1 0 0 0 .965-.738ZM1 13.116l4.778-2.867L1 7.383v5.734ZM7.059.435a2 2 0 0 1 1.882 0l6 3.2A2 2 0 0 1 16 5.4V14a2 2 0 0 1-2 2H2a2 2 0 0 1-2-2V5.4a2 2 0 0 1 1.059-1.765l6-3.2"/>
+	
+
+</svg>

static/icon_cache/save_None_None_None.svg

@@ -0,0 +1,8 @@
+<?xml version="1.0" ?>
+<svg xmlns="http://www.w3.org/2000/svg" width="16" height="16" fill="currentColor" class="bi bi-save" viewBox="0 0 16 16">
+	
+  
+	<path d="M2 1a1 1 0 0 0-1 1v12a1 1 0 0 0 1 1h12a1 1 0 0 0 1-1V2a1 1 0 0 0-1-1H9.5a1 1 0 0 0-1 1v7.293l2.646-2.647a.5.5 0 0 1 .708.708l-3.5 3.5a.5.5 0 0 1-.708 0l-3.5-3.5a.5.5 0 1 1 .708-.708L7.5 9.293V2a2 2 0 0 1 2-2H14a2 2 0 0 1 2 2v12a2 2 0 0 1-2 2H2a2 2 0 0 1-2-2V2a2 2 0 0 1 2-2h2.5a.5.5 0 0 1 0 1z"/>
+	
+
+</svg>