Elastic Search and SuperEvent integration with Indico plugins

<!--HTML-->At CTBTO, we have implemented an Elastic Search plugin, derived from the CERN Citadel plugin. As we are using Elastic Search in many areas, but we do not have a need for the Citadel middleware, this solution works great for us. SuperEvent is an event platform and a mobile app that we use for our SnT conference. We have created a LiveSync agent plugin to interface with SuperEvent API. As SuperEvent also needs to be alerted when the registration information changes, we have enhanced Indico's LiveSync plugin to include registration information.