Add OS icons for graphical desktop clients

[progval]

This doesn't add icons to Terminal Clients, so this PR does not conflict with #380

I'll add them if we decide to merge this PR before #380.

Note that the "freebsd" icon isn't displayed because we use FontAwesome 4, and it was added in FontAwesome 5.

[DanielOaks]

I don't mind this but let me look into the table changes first (it's a tiny bit unwieldy with the current layout). I may split this into a separate column depending on stickyness of the top/left headers

[progval]

Sure, no hurry

[DanielOaks]

We'll need to update FontAwesome to get the bsd icon but we can worry about that later.

For now could you add this to _swtable.scss just under where the other os support colours are:

.os-support-macos {
    color: #444;
}
.os-support-windows {
    color: #4d75b3;
}
.os-support-linux {
    color: #826d2e;
}
.os-support-bsd {
    color: #a42f2f;
}

bitchx and swirc both support all of the OSes, according to their websites/install instructions.

I think Glirc supports all of them as well?

[sadiepowell]

It's probably fine to just group BSD and Linux under a general "UNIX" category.