rename l1/l2 pay and add chain / currency sumbols to UI

[geoknee]

Closes #120

[NiloCK]

Slightly mixed minds about this - this is a tech demo for a tech audience, but a big benefit of this system is that it abstracts away "guts" from the end user, who probably benefits most from a simple "Pay" button denominated in their local currency.

I'd lean toward putting this in, but I wonder if there's a different way to emphasize the bridging / network hopping, while also demonstrating a very friendly UI to the end user.

[geoknee]

"chain pay" / "bridge pay"?

[geoknee]

And for consistency, "host chain" ("target chain" even)