chore: lint css, json, actions, html

[jniles]

Lints the .css files, json, and action files. The goal is to have better standards for each component of our code base.

In particular:

1. All github actions have pinned their versions to prevent regressions.
2. CSS files have been lightly rearranged to pass some lint rules and remove deprecated rules.
3. JSON files now have a space between their colons and the values.
4. Many, many broken HTML rules were fixed, including partial tags, unclosed tags, too many tags, etc.

This PR also installs super-linter as a Github action. Note that super-linter will continue to show a failed build while I tune the lint rules. It is not meant to be a required check, but rather serves to alert of regressions in our code.

[jniles]

All the HTML lint checks finally pass! Merging.