Resolve "User "cannot merge" an issue 🤔"

What does this MR do?

Previously, when the user was loading a design via the direct link and then closing a design, issue assignees were given a cannot merge badge. This happened due to the wrong issuableType prop: we were checking if we are on the issue page but designs page was not matching the condition, thus issuableType was set to merge_request.

This MR adds a check for design page and sets a correct issuable type for it.

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

Related to #299457 (closed)