Updates on success of an MR the count on top and in other tabs

What does this MR do?

This will update the open MR count in the top Navigation as soon as you merge or close an MR and it updates also all other open tabs (nice for tab junkies like me) without doing polling.

• When a MR is merged it requests from a new API the updated MR Count
• New API endpoint for MR count
• Updates other tabs by sending a BroadcastMessage
• When a MR is closed it requests updated MR
• When a MR is unassigned or assigned
• API specs
• JS specs

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance 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

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