WIP - Implement GeoApp for Dashboards

[mattiagiupponi]

This is a refactor to implement in a lighter way new GeoApps on map store client. The idea behind this refactor is summarized in the following points:

• Remove GeoStory model and relatives files/code
  • The GeoStory model doesn't give any kind of new information to the base-app model. In this case, all the information can be collapsed into the standard GeoApp model. The app_type is basically the resource_type in ResourceBase model. All the code specific to the GeoStory (serializer, endpoints, URLs, etc...) has been removed
• Remove legacy unused code
  • Some code related to the MapstoreAdapter models was not removed in the first round of the refactor. I have just removed some unused code.
• Refactoring of all GeoStory endpoint from the client
  • Since the GeoStoryis a GeoApp, all the endpoints from the client perspective has been updated to call the normal api/v2/geoapps instead of the specific endpoint `api/v2/geostory'
• UI changes
  • The UI has been changed to show the Dashboard in the top menù. I have also fixed the counters for GeoStory (by filtering for his specific resource_type via geo apps endpoint) and the one for the Dashboard. It's a POC, so will evolve in the future
• Placeholder app for Dashboard
  • Since I don't know how the Dashboards will be handled (if it requires some specific field), I have created a placeholder app for them to add an URL field (maybe we want to store remote dashboards?), but if it will not be required is easy to delete it
  • I also add some files and code that looks like is required to the dashboards (I replicate the one for the GeoStory) but I leave this part to the FE team since I have no idea of how to implement it.
• Compiled files

There are few changes in GeoNode too to let this refactor works. GeoNode PR: https://github.com/geosolutions-it/geonode/pull/880

[giohappy]

please @allyoucanmap give it a look from the client side along with https://github.com/geosolutions-it/geonode/pull/880