Changed trading form label to [Buy/Sell] [currency] attempt 2

This is a redo of https://github.com/stellar/stellar-client/pull/856 based on the comments from the PR. It was quicker to start from this fresh branch with a mergeable up-to-date trading.

I will close PR 856 after this gets merged in so that the comments are still there.

Implements https://github.com/stellar/stellar-client/issues/850