ci: migrate to GitHub Actions from CircleCI, allow running Browserstack on forked repo via label (#2417)

* Remove CircleCI and codecov

* Add GitHub Actions workflow

* Add Browserstack credentials

* Skip tests if ci skip or skip ci is in commit messages

* Update and rename test.yml to ci.yml

* Postfix artifacts name with browser

* Disable proxy on chrome

* Update dockers/screenshotter/screenshotter.js

* Add status badge

* Set directory option in codecov action

* Run Browserstack on forked repo via label

* Print Docker logs

* Disable GPU on ChromeDriver

* Add comments

* Add comments

README.md

@@ -1,6 +1,6 @@
 # [<img src="https://katex.org/img/katex-logo-black.svg" width="130" alt="KaTeX">](https://katex.org/)
 [![npm](https://img.shields.io/npm/v/katex.svg)](https://www.npmjs.com/package/katex)
-[![CircleCI](https://circleci.com/gh/KaTeX/KaTeX.svg?style=shield)](https://circleci.com/gh/KaTeX/KaTeX)
+[![CI](https://github.com/KaTeX/KaTeX/workflows/CI/badge.svg?branch=master&event=push)](https://github.com/KaTeX/KaTeX/actions?query=workflow%3ACI)
 [![codecov](https://codecov.io/gh/KaTeX/KaTeX/branch/master/graph/badge.svg)](https://codecov.io/gh/KaTeX/KaTeX)
 [![Discussions](https://img.shields.io/badge/Discussions-join-brightgreen)](https://github.com/KaTeX/KaTeX/discussions)
 [![Dependabot Status](https://api.dependabot.com/badges/status?host=github&repo=KaTeX/KaTeX)](https://dependabot.com)