Promote EE features

What does this MR do?

It is the basic to show promotions in EE based on `UserCallout', they are differently styled in gray (see below). I 'stole' already the URL logic for the trial from gitlab-org/gitlab-ee!2266. I have implemented a global check if we should show promotions or not, if due to any circumstance we don't ship the trial functionality we can still turn it off without reverting multiple MR's or base it on any other setting (that's why not a constant or so).

• It promotes service desk features in the project settings (#2887 (closed))
• It promotes merge request features in the project settings (#2888 (closed))
• It promotes squash commits on the MR page (#2918 (closed))
• It promotes features on the repository settings page (#2948 (closed))
• It promotes burndown charts on the milestone page (#2966 (closed))
• It promotes issue export on issues page in the standard modal (#2987 (closed))
• Activates the project audit events page and promotes it there if the feature is not available (#2988 (closed))
• Activates the group contribution analytics page always and promotes it if the feature is not available (#2967 (closed))

Are there points in the code the reviewer needs to double check?

• My checks if that functionality is available in this license
• Do we need a Changelog entry for a promotion?

Why was this MR needed?

Promote Service desk and then also other things

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• 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 #2887 (closed), #2888 (closed), #2918 (closed), #2966 (closed), #2987 (closed), #2988 (closed), #2967 (closed)