Default Export Zpub

## Issue:
* It is extraordinarily difficult to add Jade to an existing quorum of multisig devices.

## Description:
When adding a Jade to an existing quorum of multisig devices, all that is needed is the ability to export a zpub or xpub, and then later sign a PBT.

Both of these tasks are impossible to do with the Jade itself, as well as with the Green app tied to the Jade.

They only appear possible to do with the HWI interface.

This may be because in part, the multi-QR code functionality is broken and incompatible with any of the devices I tried it with.

## Desired Behavior
* A main menu option to export the device's zpub or xpub for the device's default account
* Display this z/xpub on the device so at minimum the user can type it out on their machine
* Show this z/xpub on the screen with *manual* qr code cycling as discussed in #200
* Less important: The Green app should be able to do this as well

Start with the HWI interface and the multi-QR behavior discussed in issue #200, then trace how the Jade and Green app currently expose account data. Done means the default account's zpub or xpub can be exported from the device, displayed for manual entry, and cycled through manually as QR codes; Green app support is a secondary goal.

Written by the indexing model from the issue text.