Add upload feature for metric images

What does this MR do?

Incidents (an extensible issue type) now have a 'Metrics' tab. This tab contains an upload area similar to the design management upload feature, in that users may attach images.

Users may optionally add a url to the metric image, which adds a link to the title when it is displayed.

There is a fast follow-up to allow metric images to be deleted: #291011 (closed).

Notes:

1. This feature is behind the feature flag :incident_metric_upload_ui. Run Feature.enable(:incident_metric_upload_ui) in the rails console to enable it.
2. Requires a GitLab Premium or above license.

Issue: #235994 (closed)

More info:

• Docs MR: !50044 (merged)
• Feature flag rollout: #293934 (closed)

Screenshots (strongly suggested)

Incident with empty metrics tab	URL dialog	Incident with attached metric (and url)	Collapsed metric

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation (if required) - docs MR here: !50044 (merged)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [n/a] 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
• [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