Order book loading

Show "Loading..." while the order book data is still loading.
Wrapping ```setCurrentOrderBook``` in a singletonPromise exposed the fact that when the currency changed, the issuer was still the old value in the requested order book. Although the issuer gets set immediately afterwards, the singletonPromise blocks the attempt to load the correct order book. Explicitly setting the issuer when the currency changes fixes the issue. This implicit behavior was also preventing the issuer from changing correctly.

Fixes #843.
Fixes #858.