Domains: Update the icon we use for domains in the sidebar

[breezyskies]

When we launched the domain sidebar item, globe was being used for the "view site" link. That item has now been incorporated into the site card, so this PR updates the "domains" sidebar item to use the globe icon instead of cart (since this section includes domain management, not just purchases).

Before	After

[rickybanister]

Looks right to me.

[stephanethomas]

Code looks good.

[mtias]

We use globes for default site-icons as well. It'd be good to have a more specific one for domains, but this seems good for now.

[folletto]

I think this is a good iteration up. :)

Would be even better in my view to make a new icon to Gridicons that is something like a globe with an overlaid URL/address field on top of it (signifying globe + address). :)

[breezyskies]

Would be even better in my view to make a new icon to Gridicons that is something like a globe with an overlaid URL/address field on top of it (signifying globe + address). :)

Agreed, although as @mikeshelton1503 pointed out when we were looking at this, it's going to be hard to do this in a readable way at this size. Something to think about and iterate on in the near future. :)