Allow users to change the default proxy host icon to a custom one.

[nxtlo]

Is your feature request related to a problem? Please describe.

not related.

Describe the solution you'd like

When editing the proxy host details, add a 'Set Icon' button which allows the user to set the icon either via (Uploading a file?), or a URL (Would probably be the easiest solution).

Currently this is how it looks like:

inspecting the CSS element already shows that the base image is being fetched via this URL:

and then by simply replacing the URL with a custom one:

Describe alternatives you've considered

none

Additional context

icon declaration:

nginx-proxy-manager/frontend/src/components/Table/Formatter/GravatarFormatter.tsx

Line 14 in fec8b3b

backgroundImage: `url(${url || defaultImg})`,