Promote burndown charts

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

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Design

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

Admin

• Having Start GitLab Enterprise Edition trial button.
• Read more

• Link to the document, so users can know more about the feature.
• Link: https://docs.gitlab.com/ee/user/project/milestones/burndown_charts.html

• Copy:

Improve milestone with Burndown Charts.

Burndown Charts are visual representations of the progress of completing a milestone. 
At a glance, you see the current state for the completion a given milestone. 
Without them, you would have to organize the data from the milestone and 
plot it yourself to have the same sense of progress. [Read more]

Non-admin

• Copy:

Improve milestone with Burndown Charts.

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

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 milestone with Burndown Charts.

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

Non-admin

• Copy:

Upgrade your plan to improve milestone with Burndown Charts.

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

Admin	Non-admin

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SVG: graphic-burndown-charts.svg