Reorder Issue action buttons as per UX recommendation

What does this MR do?

It reorders Issue action buttons as mentioned in https://gitlab.com/gitlab-org/gitlab-ce/issues/18927#note_30044232

Why was this MR needed?

This is an enhancement in usability by reordering action buttons based on order of importance.

Screenshots

Action Buttons

Action Dropdown

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• [ ] Documentation created/updated
• [ ] API support added
• Tests
  • [ ] Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #18927 (closed)