Resolve "Group Activity - MRs Chart w/ Single Series: App skeleton"

What does this MR do?

This MR introduces the skeleton code required for the report page. It also updates the metric titles and provides the first link through to the new page!

Screenshots

Screen_Recording_2020-06-29_at_15.06.48

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

Related Issue

#218459 (closed)

Testing

In order to test you will need to enabled the group_activity_analytics and report_pages feature flags. You can then browse to a group overview page and click the highlighted link as displayed in the description.