Promote merge request approval

Related to https://gitlab.com/gitlab-org/gitlab-ee/issues/2624

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Design

• Having a banner above Project avatar section.
• Merge request approvals, Fast-forward merge, and GitLab service desk positions are very close on the project settings page, so they are all included in the same banner.
• GitLab service desk is a feature for GitLab EEP, you can see more design on #2887 (closed).

Promote trial or license activation for regular EE on-prem sites.

Admin

• Having Start GitLab Enterprise Edition trial button.
• Having x button. --> If click it, the banner will disappear.
• Read more

• Link to the document, so users can know more about the features.
• Merge request approvals: https://docs.gitlab.com/ee/user/project/merge_requests/merge_request_approvals.html
• Fast-forward merge: https://docs.gitlab.com/ee/user/project/merge_requests/fast_forward_merge.html
• GitLab service desk: https://docs.gitlab.com/ee/user/project/service_desk.html

• Copy:

Improve Merge Request and customer support with GitLab Enterprise Edition.

[Merge Request Approvals]
Merge request approvals allow you to set the number of necessary approvals and predefine a list of approvers 
that will need to approve every merge request in a project.

[Fast-forward Merge]
If you prefer a linear Git history and a way to accept merge requests without creating merge commits, 
you can configure this on a per-project basis.

[GitLab Service Desk]
GitLab Service Desk is a simple way to allow people to create issues in your GitLab instance 
without needing their own user account. 

Non-admin

• Copy:

Upgrade your plan to improve Merge Requests and customer support.

{{ Same as admin copy for regular EE on-prem sites }}

**Contact your Administrator to upgrade your license.**

Admin	Non-admin

For GitLab.com the feature promotion needs to go to purchase a GitLab.com plan

Admin

• Having Upgrade your plan button.
• Copy:

Upgrade your plan to improve Merge Requests and customer support.

{{ Same as admin copy for regular EE on-prem sites }}

Non-admin

• Copy:

Upgrade your plan to improve Merge Requests and customer support.

{{ Same as admin copy for regular EE on-prem sites }}

**Contact your Administrator to upgrade your plan.**

Admin	Non-admin

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SVG: graphic-merge-request-purple.svg