Roadmap markup relayout

What does this MR do?

Changes Epic Roadmap markup to use <div> and <span> instead of <table> for better scroll performance and cleaner implementation

Why was this MR needed?

Current implementation of Roadmap is based on <table>, where header and first column are fixed. Problem with this approach is that position: sticky; cannot be utilized due to lack of browser support for sticky positioning on <table> elements, hence we had to set inline style left on elements which are required to remain sticky on scroll.

This MR replaces <table> in favour of <div>s and <span>s where position sticky is fully functional for Chrome, Firefox, Safari and Edge (IE11 doesn't support it and treats it as position: relative;). This allows for cleaner implementation of the layout and less hacks to deal with, leading to no side-effects as mentioned in #5068 (closed).

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• [ ] Documentation created/updated
• [ ] API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • [ ] Has been reviewed by Backend
  • [ ] Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• [ ] If paid feature, have we considered GitLab.com plan and how it works for groups and is there a design for promoting it to users who aren't on the correct plan
• [ ] End-to-end tests pass (package-qa manual pipeline job)

What are the relevant issue numbers?

Closes #5068 (closed), #4848 (closed)