"Install software update" button posts, shows static "installing" page

...which redirects to the main app after 30 seconds (hopefully enough
time for the installation to complete & the appliance to reboot).

The spinner gif was generated from http://preloaders.net/en/circular
(foreground color: #f77f00; size: 16x16). The license allows commercial
use without mentioning the source:

> Terms of use
>
> * You are encouraged and legally entitled to copy and modify any
>   animated GIF and APNG images, generated by http://preloaders.net
>   web-site.
> * All animated GIF and APNG images are completely free to use in all
>   projects (web and desktop applications, freeware and commercial
>   projects)
> * You are encouraged, but NOT obligated to place any reference to
>   http://preloaders.net web-site to use it`s content in any projects
>   and applications, WITH THE EXCEPTION of any other animated images
>   generator applications, which must indicate the source of the used
>   image with a visible link to Preloaders.net web-site on one or more
>   pages that is/are linked to from the main page of the web-site that
>   uses the image from Preloaders.net web-application
> * Preloaders.net project reserves the right to refuse service to
>   anyone without any explanation and/or notification