Add icon for windows installer download button

Icon from https://www.iconfinder.com/icons/9026548/download_thin_icon - MIT license.
In accessibility options, the link reads as `download Windows installer`

Signed-off-by: Egor Savkin <es@m-labs.hk>

static/images/icons/icon-download.svg

@@ -0,0 +1 @@
+<svg version="1.1" viewBox="0 0 256 256" xmlns="http://www.w3.org/2000/svg"><g transform="matrix(1.1429 0 0 1.1429 -18.286 -4.5719)" stroke="#fff"><g fill="none" stroke-linecap="round" stroke-linejoin="round" stroke-width="15.999"><path d="m176 128h48a8 8 0 0 1 8 8v64a8 8 0 0 1-8 8h-192a8 8 0 0 1-8-8v-64a8 8 0 0 1 8-8h48"/><line x1="128" x2="128" y1="24" y2="128"/><polyline points="80 80 128 128 176 80"/></g><circle cx="188" cy="168" r="8" fill="#fff" stroke-width="3.9994"/></g></svg>

templates/index.html

@@ -53,7 +53,10 @@
               <a href="{{ get_url(path='@/experiment-control/place-order.md') }}" class="btn btn-primary btn-inversed btn-lg">Order hardware</a>
               <a href="https://m-labs.hk/artiq/manual/" class="btn btn-primary btn-lg">Manual</a>
               <a href="https://forum.m-labs.hk" class="btn btn-primary btn-lg">Forum</a>
-              <a href="https://nixbld.m-labs.hk/job/artiq/extra/msys2-offline-installer/latest/download/1" class="btn btn-primary btn-lg">🖴 Windows installer</a>
+              <a href="https://nixbld.m-labs.hk/job/artiq/extra/msys2-offline-installer/latest/download/1" class="btn btn-primary btn-lg d-inline-flex">
+                  <img src="/images/icons/icon-download.svg" class="d-inline-block align-self-center mx-1" style="height: 1rem" alt="download">
+                Windows installer
+              </a>
               <a href="{{ get_url(path='@/experiment-control/artiq.md') }}" class="btn btn-primary btn-lg">More...</a>
 
             </div>