Change 'Datasets' field on Resources form to use autocomplete

[ghost]

Issue: CIVIC-6574

Description

(NOTE: This PR is a redo of #1996 to include it on 1.13.6)

The dataset field on the resources form is currently listing all the available options which on sites with a big amount of datasets can be an issue. We've modified the field to use autocompletion, the same way we are doing now the dataset form to add resources.

This PR includes:

• Changes on 'dkan_dataset.forms.inc' and 'dkan_dataset_content_types.features.field_instance.inc' to implement the fix.
• The 'iFillInTheResourcesFieldWith' custom step was used to fill in the autocomplete 'Resources' field on the Datasets form. We needed the same kind of step in order to fill in the 'Datasets' field so the custom step was renamed to 'iFillInTheAutocompleteFieldWith' and moved out from 'DatasetContext' to 'DKANContext'.
• Several Behat tests were fixed in order to use the new step name and/or to support the new autocomplete field.

QA Steps

• Go to create a resource.
• Confirm that you are able to assign it to any dataset using the autocomplete form.

Reminders

• There is test for the issue.
• CHANGELOG updated.
• Coding standards checked.
• Review docs.getdkan.com (or in /docs) to see if it still covers the scope of the PR and update if needed.

[dkinzer]

I reviewed the dataset components docs and I don't think we need to make changes based on this PR. QA steps are met ad tests are passing so I'm merging.