Fix #815 and reduce subscription trashing on trading

This commit makes a couple of changes to the trading controller:
-  It fixes a bug where we were making a subscribe request
   where the currency was STR and we were supplying an issuer as well
-  It refactors how we check validity of the currency selector form so
   that we don't make 3-4 subscribe/unsubscribe calls to stellard as
   the initial page is loaded
Merge pull request #818 from stellar/stellar-client-815

Fix #815 and reduce subscription trashing on trading