Highlight component's decription should be wrapped in <p></p>

[mmmavis]

Context:
See ##4263 (comment)

Highlight component's description is a TextField but currently rendered as rich text. We should make sure the description is wrapped in <p></p> for semantic purposes and for the right style to kick in.

Relevant code to look at
https://github.com/mozilla/foundation.mozilla.org/blob/master/network-api/networkapi/wagtailpages/templates/wagtailpages/tags/card.html#L14
Since this template is used by different components, we shouldn't assume description is always in rich text format.