Improve withdraw experience & handle splits

[efstajas]

There's a few things we can do to improve UX on top of #116, which adds a basic withdraw button to the balance dropdown:

• If the user has splits set up on their Drips account (which they could do through the drips app), the withdraw will currently fail. Instead, if should probably show the user who will receive what amount, and how much they will receive themselves, and then initiate the withdraw including the splits.
• While the transaction is waiting for wallet approval, we should give the user feedback to check their wallet to approve.
• While the transaction is pending, we should give the user feedback that it's pending.
• Once the withdrawal is complete, the withdraw-able amount should automatically go down to 0.