EE port: Make external_dashboard_url available to frontend

What does this MR do?

If a project has a grafana url persisted in the DB (feature added in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/27880), this work will surface that URL to the metrics dashboard frontend. Along with some additional frontend work, this will allow users to click a button through to grafana for the given environment & view further metrics.

EE port of https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/28151

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