templates: add an opaque icon for mobile home screens

The netbox_touch-icon-180.png icon was produced by rendering
netbox_icon.svg into a 160x160 square, centered in a 180x180 PNG filled
by the background colour of #212529.

In other words, it is a screenshot of the following HTML element:
```html
  <div style="width: 180px;height: 180px;background-color: #212529;">
  <svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 320 320" style="padding: 10px;">
    <g fill="#9cc8f8" stroke="#9cc8f8">
      <circle cx="37" cy="284" r="23"></circle>
      <circle cx="101" cy="37" r="23"></circle>
      <circle cx="101" cy="220" r="23"></circle>
      <circle cx="284" cy="220" r="23"></circle>
      <rect x="93" y="37" width="16" height="180"></rect>
      <rect x="101" y="212" width="180" height="16"></rect>
      <rect x="93" y="212" width="16" height="90" transform="rotate(45 101 220)"></rect>
    </g>
    <g fill="#1685fc" stroke="#1685fc">
      <circle cx="284" cy="37" r="23"></circle>
      <circle cx="37" cy="101" r="23"></circle>
      <circle cx="220" cy="101" r="23"></circle>
      <circle cx="220" cy="284" r="23"></circle>
      <rect x="37" y="93" width="180" height="16"></rect>
      <rect x="212" y="101" width="16" height="180"></rect>
      <rect x="212" y="93" width="16" height="90" transform="rotate(225 220 101)"></rect>
    </g>
  </svg>
  </div>
```

netbox/templates/base/base.html

@@ -124,6 +124,7 @@
       onerror="window.location='{% url 'media_failure' %}?filename=netbox-print.css'"
     />
     <link rel="icon" type="image/png" href="{% static 'netbox.ico' %}" />
+    <link rel="apple-touch-icon" type="image/png" href="{% static 'netbox_touch-icon-180.png' %}" />
 
     {# Javascript #}
     <script