Replace PhantomJS with headless Chrome in frontend test suite

What does this MR do?

• adds karma-chrome-launcher to devDependencies
• removes karma-phantomjs-launcher from devDependencies
• replaces PhantomJS with ChromeHeadless browser config in karma.config.js
• fixes failing tests
• remove phantomjs-specific hacks from tests
• updates .gitlab-ci.yml to utilize new docker image containing headless Chrome

Are there points in the code the reviewer needs to double check?

• karma test job should pass

Why was this MR needed?

see #30876 (closed)

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Tests
  • Added for this feature/bug
  • All builds are passing
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

part of #30876 (closed)

related: gitlab-build-images#10 (closed)