Removed CommunicationManager and PortletCommunicationManager

* Moved AbstractCommunicationManager abstract methods
  getThemeResourceAsStream and createBootstrapHandler to VaadinService
* Made ACM non-abstract and renamed to LegacyCommunicationManager
* Lifted anonymous inner BootstrapHandler subclasses into named public classes

Change-Id: I31739ce8a506d572e75ca8cd5509be215e01693d