Convert SCSS variables into hyphen case

This makes all the variables consistent with each other and follows
CSS language conventions of lowercase with hyphens separating words.

Also updated form module to use our css color variables.
Remove ng-valid styles and use error-red on forms
Merge pull request #1351 from hypothesis/scss-variables

Convert SCSS variables into hyphen case
Fix spacing between icons and text inside buttons, improve spacing on annotation cards, add placeholder for annotation creation textarea, adjust padding on tags.
Add examples to pattern library, polish CSS and HTML in accordance to @aron's suggestions.