Studio: HTML &lt;body&gt; class and view-styling naming conventions

This work revises the class naming conventions applied to the <body> element which are used to scope view-specific styling and presentation within our Sass/CSS. This originated from @frrrances good feedback here - https://github.com/edx/edx-platform/pull/1043#discussion_r6696043.

No styling was changed, just selector an attribute names.