Prevent adding multiple transaction listener

Multiple transactions were showing up in the history, because each time the app navigated to the dashboard another transaction listener was added.

 - Remove ```transactionHistory.init()``` in favor of ```transactionHistory.getPage()``` ensuring that the service is initialized before continuing.
 - Wait for ```StellarNetwork.ensureConnection()``` before initializing instead of waiting for an event.
 - Remove ```cleanupListeners()```. Since ```init()``` is now guaranteed to only run once and we are no longer initializing after every reconnect, we never have to remove the transaction listener.

Fixes #817.