Split the dashboard header to a separate component

What does this MR do?

Spring cleaning. ☀ Splits the dashboard header to a separate component. #215465 (closed)

The dashboard component is too complex at the moment, this is a "minimum change" to split the dashboard header to its own component.

• it's almost a cut and paste of the header html (and required props & methods) into a new component.
  • snapshot didn't even change! (save for spacing differences)
• all specs (except one) are kept the same, the header component is not stubbed
  • this ensures minimum changes to the specs, keeping the same features.
  • so no splitting to create dashboard_header_spec yet!
• a few events were added for transporting data between the two components

Moving forward

A lot of the props of the new dashboard-header could be replaced by checking the Vuex store or by other cleaner means, this first step does not consider these changes.

Also dashboard_header_spec should be created out of the specs that use .find(DashboardHeader) finders in the future.

Screenshots

N/A

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