[meta] Add in js- prefixed classes for separation of JS & CSS functionality

Problem/Motivation

We expose many class attributes in templates and other layers (for example preprocess), but it's never clear which classes are needed for JavaScript functionality, leaving themers in the dark if they want to slim down their markup and not break JS.

Proposed resolution

Add CSS classes for JavaScript functionality. Classes will use the original name and be prefixed with .js- and be added to the existing class (in Classy) for separating design and functionality.

Classy

<div class="foo"> -> <div class ="js-foo foo">

Core

<div class="foo"> -> <div class ="js-foo">

This is a step along the way to using data attributes, but even if we don't get to data attributes it's still an improvement.

Remaining tasks

See child issues.

User interface changes

None for themes extending Classy. Possible visual changes to themes not extending Classy.

API changes

n/a

Beta phase evaluation