ODM: Implement widget for visualizing datasets grouped by metadata field

[acorbi]

As part of milestone 2.1, a widget should be created which would receive the name of a metadata field on its configuration, will then query the available datasets and group them in a visual compelling form, making possible to browse through them, making them discoverable.

• Research and choose a visualization style
• Implement widget configuration with following options:
  - Title
  - Metadata field
  - Possible options
• Implement dataa representation with calls to CKAN API