Fix error when creating labels in a new issue in the boards page

What does this MR do?

This MR fixes a problem when adding labels to a newly created issue from the boards page (more info in the linked issue).

After some debugging I found out we weren't adding assignableLabelsEndpoint to the issue after creating it like we do with some other values so the data-labels attribute wasn't added.

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/54311

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer

Closes #54311 (closed)