Create a different state for the swap screen when no trustlines exist

[sdfcharles]

When swapping, and the account doesn't have any trustlines, we should display a different screen and redirect the user to the asset manage screen.

[JakeUrban]

Maybe you're already accounting for this, but only stellar assets have trustlines (you don't need trustlines for custom tokens), so the check should be whether the user has added any assets to his/her wallet at all.