[Priority: Low] Add sending all coins

[nopara73]

A user cannot send all its coins out of the wallet without leaving some small amount in, because of the fee calculation, it should be supported.

Illustration:

[bokobza]

This is already implemented in the wallet back-end. There is a method called GetMaximumSpendableBalance in the WalletController.
I think @dev0tion will add it to the UI when he's done with the TumbleBit work.

[nopara73]

Is it exposed to the HTTP API? If yes, could you insert the call here?

Are you sure it'll work correctly?
He might gets the maximum spendable balance, then makes a build transaction and it throws an error: not enough fee?
It should've be rather build-transaction, parameter amount: all.

[dev0tion]

Max balance function added to the frontend logic in #279
Have yet to figure out a way to display it in the current coin unit.
Should be resolved in the next sprint.