`js-` prefix; fix content preview back button

Closes #1990, Ref #3810
- Add `js-` prefix to jQuery calls throughout code and templates
- move MarkdownComponent classNames to template
- Fix back button on posts content preview
- Remove dead code for preview tabbing jQuery
Merge pull request #3949 from novaugust/js-prefixes

`js-` prefix codebase; fix content preview back button