templates: add an opaque icon for mobile home screens

[bonktree]

Fixes: #8103

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:

  <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>

The child <svg> is the contents of netbox_icon.svg.

[bonktree]

iOS devices fill the transparent background of an icon with #000000, and the light blue stroke of NetBox's logo is hardly legible on a white background, so I decided to go with the colour NetBox uses in dark mode instead.