* Split BuyAmountInput out of AmountInput
* Buy and sell tabs. Compute some sell info
* In progress
* BuyPanel & SellPanel with banner above that shows current shares and toggle button
* Remove "Remaining balance"
* Bring back 'Place a trade'. Tweaks
* Sell shares cloud function.
* Sell all shares by default. Switch back to buy if sell all your shares.
* Cache your shares in local storage so sell banner doesn't flicker.
* Compute sale value of shares with binary search to keep k constant.
* Update bets table to show BUY or SELL
* Fixes from Stephen's review
* Don't allow selling more than max shares in cloud function
* Use modal for sell shares on desktop.
* Handle floating point precision in max shares you can sell.
