Fix design target note type value in activity stream

What does this MR do?

See #219380 (closed)

This fixes the formatting of note_target_type for events on notes on designs, by adding a new method to produce a human readable value.

The test is updated to avoid a previous false positive.

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