CE port of updated welcome page

What does this MR do?

This MR brings changes from https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/2301 where we updated welcome pages for GitLab Admin user as well as standard user based on UX provided in https://gitlab.com/gitlab-org/gitlab-ee/issues/2501.

Why was this MR needed?

It provides better first impression to new GitLab user.

Screenshots

Admin User

Desktop View

Tablet/Mobile View

Standard User

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?

https://gitlab.com/gitlab-org/gitlab-ee/issues/2501