Resolve "Improve default title and description of issues opened from managed Prometheus alerts"

What does this MR do?

This MR improve the default title and description of incident issues by

• parsing mandatory labels/alertname as title
• parsing and exposing alerts' mandatory start time (via starts_at)
• parsing and exposing alerts' full query (via generatorURL)

Despite these improvements this MR also

• moves alert summary creation logic into the alert presenter
• refactors several speccs
• uses rfc3339 consistently

Please see individual commit messages to watch tiny, incremental steps.

Refs #11212 (closed)

Screenshots

Minimal alert with only labels/alertname
Average alert
GitLab-managed alert

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