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
Remove the drop shadow from the highlight states
Improve visibility of nested highlights