Highlight row when alert is new

What does this MR do?

We want to highlight table rows when items are new, similar to how the issues/merge request lists currently do.

This MR implements this change.

To test locally: Ensure you have some alerts with a start date within 12 hours of the current time. Sending an alert with no start time (see the example at https://docs.gitlab.com/ee/user/project/integrations/generic_alerts.html#customizing-the-payload) will result in one with a start time of now, fulfilling the requirement to see the 'new alert' highlight.

Issue: #213899 (closed)

Screenshots

before	after	after (hover effect)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [n/a] Documentation (if required) Documentation not required as it's a minor change? Happy to go either way
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [n/a] Database guides
• [n/a] 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
• [n/a] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

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:

• [n/a] Label as security and @ mention @gitlab-com/gl-security/appsec
• [n/a] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [n/a] Security reports checked/validated by a reviewer from the AppSec team