Hero styling is not properly scoped

Problem

Some hero styles are declared in hero.scss that affect the page title. However, this is set to a generic page title selector and not scoped to the hero block. This causes issues if a page title element is displayed outside of the hero while the hero css is loaded on the page.

Solution

The cover class is consistently used as top level class for hero elements. Ideally we'd use a new class for the scoping but that requires editing a lot more heroes and could break other custom hero elements that live outside of Open Social. So to fix this the .page-title class (and other generic classes) are scoped to .cover if they're not already scoped to a hero specific element.