Code cleanup after horizon migration

[TbLtzk]

• remove everything that remains from old websocket
• remove websocket plugin
• call other plugins into question
• Use Account.isValidAddress()
• call healthCheck into question
• analyze upgrade to higher versions of
  • ionic
  • cordova

[TbLtzk]

part of websocket stuff is removed but still more to investigate: https://github.com/raymens/Centaurus/blob/master/www/js/services/services.basic.js#L108-L133 which should not be needed anymore. The healthcheck might be useful later but at the moment is not, because isConnected() always returns true.
Might check the goodbye-stellard branch to see how it used to be..