Generalized json-widget to handle non-config fields

[redhog]

class MyForm(BaseForm):
    class Meta(BaseForm.Meta):
        widgets = {'my_field': JsonSchemaWidget(attrs={
                                                     # Only use one of these two options at a time
                                                     'data-schema': 'my-schema',
                                                     'data-schema-selector': '#my-dropdown-field',
                                                     # Options for the json-editor
                                                     'data-options': json.dumps({
                                                         "disable_collapse": False,
                                                         "disable_edit_json": False,
                                                         "display_required_only": True
                                                     })})}

The diff looks bigger than it actually is, since it unindented a whole block one level...

[coveralls]

Coverage remained the same at 100.0% when pulling f9381be on innovationgarage:generalized-json-widget into cd7fbb4 on openwisp:master.

[nemesifier]

@redhog sorry for the delay, thank you for contributing 👍