Refactor code to match current conventions

This is an update to Vellum such that the CSS is written in a way that follows our current standards and conventions.

Status: **Opened for visibility**
Reviewers: @kpeatt @ry5n 
JIRA: **CSOPS-846**

## Changes
* Set headings to standard capitilization
* Max of three `=` or `-` when underlining headings
* Fixed property orders to match our linting rules
* Repositioned line comments
* Corrected line comments to be written using the numbered notation
* Cleaned up unnecessary third level headings
* Removed any placeholder classes
* Added a few extra headings for clarity
* Tried to make the variable's introductory bullet list a bit more readable
* Disable `*::before/after` by default due to Android 4.1.x performance issues
* Add `[hidden]` attribute styles (moved over from Stencil)

Seems backwards to me. We're setting default values for heading levels and then making it easy to extend those out into classes if you want to. This way, you'd have to go back into your defaults and hook them into components.