Record onboarding for issues closed by MRs

What does this MR do?

Part of: gitlab-org/growth/team-tasks#247 (closed)

This records an action when within an onboarded namespace an issue got closed via a merge request. This is part of multiple onboarding progress actions that get tracked.

No changelog since this is developer focused so far and we don't do anything yet with the recorded action.

How to test

• Create a MR within a namespace with your_namespace_id or create a new namespace
• Create an Issue
• Put Closes #<the_issue_id> into the MR description
• OnboardingProgress.onboard(Namespace.find(<your_namespace_id>)) to onboard if it's not a new namespace
• Merge MR
• OnboardingProgress.find_by(namespace_id: <your_namespace_id>)) to see that it recorded the action

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• 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
• 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:

• 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