Allow transactions to be zapped from the UI

[mslipper]

Currently, zapping a transaction requires users to use the CLI. Since it's likely that transaction fees will vary significantly in the early days of the network, it'd be helpful if users could zap transactions directly from the UI.

I'm thinking some kind of drop-down menu on the accounts screen with a 'Zap' menu item should be sufficient. When 'Zap' is pressed, a confirmation modal should appear explaining how zapping works and how it isn't guaranteed that the transactions won't be mined.

Will close #99.

[mslipper]

Updates on this:

• Stick a "Zap All" link in settings, that still shows a modal that lists all pending transactions and a warning that zapping isn't guaranteed.

cc @pinheadmz