Resolve "Promote issue board"

What does this MR do?

It adds a promotion block to the issue board as seen below if one of the features is not available for the user and the show_promotions? rules apply. The basic rules of how and what to show for promotions were already established & extensively reviewed here : gitlab-org/gitlab-ee!2540 This MR is following and applying the same patterns here.

It was already reviewed by UX (see below)

Why was this MR needed?

To promote our awesome EE features to all the people who don't have them yet.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• 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?

Closes #2899 (closed)