Refactor thumbnail creation service

[giohappy]

The current creation of thumbnails for layers and maps is quite a mess, and the following points are especially problematic:

• there are multiple implementations of thumbnail rendering of layers. At least the following two are implemented:
  • create_gs_thumbnail_geonode which apparently is only used by geoserver async signals signals
  • layer_thumbnail
• both layer_thumbnail and map_thumbnail use the pritng Geoserver module for the actaul rendering. This module is stale and quite a black box. Probably this code reflects the module that we're bundling inside the Geoserver build for GeoNode
• the creation of the thumbnail for a map apparently takes different paths in case the thumbnail is generated from the Django template pages or from the "Save" action inside the GeoNode MapStore client. Different thumbnails (different zoom level and positioning) are generated for the two cases

This module will be in charge of:

• provide a unified interface for the GeoNode components that trigger the creation of a thumbnail:
  • post layer creation tasks
  • map saving
  • metadata wizard thumbnail widget
  • set thumbnail action from layer and map detail page
• receiving a configuration for the map to be rendered (bbox, dimensions, etc.)
• delegating to specialized classes the retrieval of the layers images (WMS, XYZ, etc.)
• composing the final rendered image