Use burnup data to generate burndown charts

What does this MR do?

1. Use burnup data to generate burndown charts (if burnup_charts feature flag enabled).
2. Add toggle to load old data. Add a boolean flag to ensure we only fetch burndown data once
3. We now also render the charts before making the network request for both cases, which speeds up the rendering of charts

Screenshots

burnup_charts feature disabled	enabled

Screen_Recording_2020-09-15_at_3.18.43_pm

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team