Refactor send pane

Allows the user to send from any currency balance they hold so long as there is a path from that currency to the select currency to send.

Refactor:
- complete re-write of send controller
- Breaks send pane into 3 different templates (send form, confirm, result)
- Adds SendController as a parent scope for all three
- Adds SendFormController as the controller for send form

Why is this called `showPaths`?  Shouldn't is be named something like `allPaymentDependenciesMet`, given that is how you describe its behavior?