Integrated Alerting: Severity levels are color-coded

User Story:

As a PMM admin user, I want to be able to see different severity levels with different colors.

Acceptance Criteria:

• Colors for the severity level in the Severity column match those in the design (to be provided);
• disabled alerts severity should not be colored (black text);

Implementation Details:

• code and PRs get pushed to percona-platform/grafana repository
• test coverage
• honor light/dark mode