[#189] Implement cascade resources in the dashboard

* defines cascadeResource and cascadeNode models, so they can be retrieved from the backend
* implements draft UI to create and fill cascade resources in the data tab (temporarily)
* implements a new cascade question type
* allows users to create a cascade question and select a cascade resource
* can request publishing of cascade resource in the actionRestService in the backend
* defines strings needed for UI
[#189] implement cascade kinds in backend

* creates new kinds CascadeResource and CascadeNode
* creates dto's, dao's, payload classes and rest service to handle them
* adds a method to ActionRestService to request publishing a resource by flow services
* adds an datastore index for cascadeNodes
* adds kinds to webapp-index
[#189] Add cascade resource to survey xml

* adds Level and Levels kind to xml definition
* lets surveyAssemblyServlet add the cascadeResource information to the xml
Merge branch 'develop' into feature/189-cascading-questions
[#189] move cascade publishing to surveyUtils

* also change way post request is done
[#189] include version in post request

* and send all as strings
[#189] Centralize cascade resource ID creation

* Format code
[#189] Include cascade data in bootstrap files

* Download resources and store the data
* Handle common resources in zip file creation
[#189] Use IOUtils to copy byte array
[#189] clean up and restructure

* some modifications to incorporate Loïc's UI code
* introduces three column - navigation
[#189] merge develop into feature/189-cascading-questions
[#189] WIP - Enable exporting cascade values splitted in columns
[#189] WIP - Cascade level names for column heading
[#189] implement cascading question UI Loïc
Merge branch 'feature/189-cascading-questions' of https://github.com/akvo/akvo-flow into feature/189-cascading-questions
[#189] Sync property name with CascadeResource
[#189] Change property name to levelNames
[#189] placeholder cascade upload UI
Merge branch 'feature/189-cascading-questions' of https://github.com/akvo/akvo-flow into feature/189-cascading-questions
[#189] WIP - Exporting level per column

* Changed the JSON serializing to ObjectMapper as the current JSON.org
  implementation does not support serializing List<String> members