Allows GitLab-owned service users to bypass spam

What does this MR do?

This MR allows GitLab-owned service_users to not be checked for spam. This MR is a follow-up to !42905 (merged). See the discussion here for additional context gitlab-org/quality/triage-ops#582 (comment 430365940)

Additional Context

The main problem this MR is trying to solve is that GitLab-owned automation is being prevented from certain API actions, such as updating an Issue's description, because it is detected as spam. The previous MR !42905 (merged) added bots to the allowlist, but it turns out that bots do not have access to the API. This MR specifically adds GitLab-owned users of type service_user to the allowlist.

Screenshots

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