Fix send performance

[n13]

Fix send screen responsiveness (closes #431, closes #432)

Summary

• Click amount field immediately after entering destination - removed wait for checkphrase (this was a bug)
• Send button is always available even if we don't know the fee
• Calculating estimated fee when view opens so we know the max amount we can send
• Send uses a custom bottom sheet that manages keyboard show/hide so it doesn't bounce around like crazy when the keyboard changes from alphanumeric to decimal.
• Alphanumeric keyboard fixed for address entry, e.g. no preview, no spelling correction and so on

Details

The performance improvements are straightforward, just rearranged fetch estimated balance etc

The biggest issue was the keyboard. The send screen shows 2 text fields, one with an alphanumeric keyboard, the other with a decimal number pad.

The OS will do a little keyboard up/down animation when it's switching, but the component would react and move up and down too, very disturbing on this screen.

Turns out this is non trivial to fix on a bottom sheet, which also needs to handle a hardware keyboard (no screen keyboard), switching between hardware keyboard and on-screen keyboard, and different keyboard heights (potentially although not on iOS)

In part this is because the bottom sheet component and scroll views have some automatic keyboard detection built in. Fixed now though.