Set button label when picking an option from status dropdown

What does this MR do?

Fixes an issue with the status dropdown. It was not setting up the correct label after picking an option

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/18536

Screenshots (if relevant)

Previous

After

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together