Default to sensible panel for grafana embeds

What does this MR do?

For the sake of simplicity, the ability to embed grafana charts in GitLab Flavored Markdown was implemented with the requirement of the grafana url including the panel id of the chart to embed. See more about the feature here: https://docs.gitlab.com/ee/user/project/integrations/prometheus.html#embedding-charts-via-integration-with-grafana-http-api

This MR adds sensible defaults for the chart in case the user has forgotten to include the panel id. If no id is provided, the first valid panel on the specified dashboard will be used. If no valid panels are listed in the dashboard, no embed will render.

Screenshots

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

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