Add Burndown Charts to CE

Description

Add burn down charts to the CE version of gitlab similar to the charts available in EE. The addition of the charts would very useful to the community and can show really well in the UI. Having this feature in CE will help current users of other Agile web tools transition to the gitlab ecosystem.

(While #13422 (closed) was closed as a dup of the EE burndown feature (gitlab-org/gitlab-ee#91) this issue is being created per @markglenfletcher's comment to @MDXDave in gitlab-org/gitlab-ee#91 as we are still missing burndown charts in CE)

Proposal

Add burndown charts to the CE version of Gitlab.

Links / references

Completed EE feature gitlab-org/gitlab-ee#91

Documentation blurb

If the implementation follows the general design of the EE implementation no additional documentation needed as it presently exists for EE.