Implement ci-runners: Incident Support: Database dashboard

Implements another ci-runners incident support dashboard from the list at gitlab-com/gl-infra/scalability#940 (comment 531009765). This time the Database one.

As the dashboard is re-using a panel that was already defined for stage-groups/continuous_integration.dashboard.jsonnet dashboard, required definitions were extracted to libsonnet/stage-groups/verify-continuous-integration. From here they are re-used in both dashboards. The final appearance of the panel in stage-groups/continuous_integration.dashboard.jsonnet is unchanged.

The preview can be found at:

• ci-runners/incident-database.dashboard.jsonnet: https://dashboards.gitlab.net/dashboard/snapshot/Ra5YyPyV3nGeTL6mRG0Fb8ejnC65zpvN?orgId=1&refresh=1m
• stage-groups/continuous_integration.dashboard.jsonnet: https://dashboards.gitlab.net/dashboard/snapshot/PI2uvY7QvxGRapWXcC3UbrM4A6FWA5Rd