Add the ability for content managers to add custom classes to paragraphs

[finnlewis]

In a technical drop-in call Konrad asked about adding a custom class to a paragraph in microsites.

So this might not actually be microsites specific.

We could add a custom field to paragraphs and update paragraphs templates to add the class, if present.

We could look at a module like this: https://www.drupal.org/project/paragraphs_class

@markconroy I wonder if the approach the module takes might be good?