Fetch balances from Metamask and display in upper-right corner

[ErikBjare]

Instead of viewing which network the user is connected to we should show any available ether balance.

If Metamask isn't detected, show a "connect wallet(s)" button. This button should open a modal with instructions for how to install Metamask (and in the future other wallets).

When detected, it should show the ether icon with the balance next to it, on the next line also show the dollar amount.

In the future, when more wallets can be connected, add a + icon to connect more wallets.