Add CSS & JS global flags to represent browser and platform

What does this MR do?

This MR adds following flags to GitLab dynamically using browser gem;

• CSS Flags: CSS flags are applied to body element.
  • Browser: Browser flag is gl-browser-*, where * can be; chrome, firefox, safari, opera, edge or ie. When no match is found, it is generic.
  • Platform: Platform flag is gl-platform-*, where * can be; windows, mac, linux, android or ios. When no match is found, it is other.
• JS Flags: JS flags are applied to global gl.client object.
  • Browser: Browser flag is boolean is*, where * can be; Chrome, Firefox, Safari, Opera, Edge or Ie.
  • Platform: Platform flag is boolean is*, where * can be; Windows, Mac, Linux, Android or Ios.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer

Closes #50013 (closed)