apps.webauthn.boot() does not need an if-condition because it's only
called from session.py when the usb interface is enabled
This means that they do not need to be stored in RAM at all. The obvious
drawback is that we need to hand-edit the if/elif sequence, but we don't
register new handlers all that often so 🤷
They now live under trezor.ui.components.tt. Later
trezor.ui.components.t1 will be added and application code will be
rewritten to not use them directly in order to work on both TT and T1.
The maximum amount is defined in [Ripple's docs](https://xrpl.org/basic-data-types.html#currencies) but it's most likely in XRP and not in drops ("satoshi" equivalent). This commit increase the value to the drops equivalent.
closes#327