Standardize all CSS classed to be applied using "class: foo,bar" instead of square brackets

[technopagan]

Right now, some CSS classes are set the clean, "correct" way of using "class: foo,bar" on top of a slide inside the MarkDown syntax, but others are set using square brackets like ".cssclass[text content]".

This is inconsistent, difficult to remember and error-prone. Let's work through the CSS to make all classes be available via the correct "class: foo,bar" syntax.