Use associated_class to render belongs_to links

Created by: DevinFrenze

Use associated_class to render belongs_to links

The class_name option that allows an associative field to render the proper class / dashboard was not previously accounted for when validating whether or not to link to a belongs_to resource in the belongs_to index and show partials.

The following actions were taken:

• Add documentation for class_name in customizing_dashboards
• Make associated_class a public method for associative fields
• Call valid_action? with associated_class in belongs_to partials
• Test that valid_action? is called with correct value
• Test that the link is rendered when action is valid

Note:

• When running appraisal install, gem pundit was added several times to the gemfile
• The test that is failing is the same test that is failing on master
• This PR follows up on https://github.com/thoughtbot/administrate/issues/1050