Frontend for trial and license purchases inside GitLab EE

What does this MR do?

This MR adds frontend changes as proposed in #2501 (closed)

Screenshots

Welcome Page (Admin User)

Desktop View

Tablet & Mobile View

Welcome Page (Standard User)

Desktop View

Tablet & Mobile View

License Details

Missing License

Trial License Banner

Start EE Trial Form

Desktop View

Tablet & Mobile View

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

#2501 (closed)