Variable cdn path for icons, as per fonts

[thornyweb]

Create and use variable for icons path rather than being hard coded.

Also change a couple of hard coded font paths to use the cdn variable that already exists.

[msftclas]

All CLA requirements met.

[mikewheaton]

Thanks for the PR, @thornyweb! Looks great to me.

[mikewheaton]

The build issue is caused by the Travis config specifying an outdated version of Node. This has been fixed on the Fluent branch (see this commit) but not yet on master. I'll make a separate PR to fix it so we can get a clean build here.

[thornyweb]

Thanks @mikewheaton