Place a sell order

[jvandenaardweg]

Created PR for issue: #3

@adis-me already did some work on it, that's awesome! Still a few major things for this one:

• Use correct sell flow (see below)
• Use correct summary data (fee calculation etc..)
• Ability to change balance and trading pair from the sell modal if the user already selected those options
• Test if sell order works
• Write tests for sell modal

I think this is a correct flow to use:

1. User presses [New sell order]
2. System shows modal with option to search/select coin in balance
3. User searches/selects a coin in his balance
4. System shows option for trading pair
5. User selects to what pair he wants to trade: BTC, ETH or USDT (depending on exchange pairs)
6. System shows sell modal
7. User places sell order
8. System closes modal and shows new order in Orders page