Prefix all HTML classes

[JDGrimes]

We prefix all of our main HTML classes (like .wordpoints-hook-reaction), but not the modifier classes (.new, .edited) or sub-element classes (.view, .action). I've considered possibly prefixing them all before, but I figured the potential for conflicts with other plugins or core woudl be reduced because we'd never reference these independently, they'd always be qualified with the prefixed class in some way. However, that only prevents us from braking most other stuff, it doesn't prevent it from breaking our stuff (see Automattic/Edit-Flow#351 (comment)). Prefixing all of the classes is long, it is ugly, and it isn't fair. But in any other place in code, we wouldn't dare fail to prefix, so why here? It will make things more robust, resulting in a better UX for our users. Even if it is painful for me.

The main question will be how to handle back-compat here, I guess there might need to be a transitional period where we use both the prefixed and unprefixed things everywhere, before we remove the unprefixed versions entirely. Although during that transitional period any conflicts will continue.

[JDGrimes]

Related: WP#40686.