Iterations dropdown component is not consistent with gitlab/ui

Summary

The Iterations dropdown component currently doesn't conform to https://gitlab-org.gitlab.io/gitlab-ui/?path=/story/base-dropdown--with-header-and-footer

Current	Expected (disregard input width)

Steps to reproduce

• Navigate to an issue
• Click Edit next to Iteration in the sidebar
• View the dropdown

What is the current bug behavior?

The Iteration dropdown is missing some styling

What is the expected correct behavior?

🎨 Expected design:

• ✅ When no option is applied, placeholder in the sidebar should read None, similar to milestones, instead of No iteration
• ✅ Add a divider under the dropdown header
• ✅ Add a divider under the no iteration option

• Spacing between the Milestone and Iteration rows should be 16px/1rem
• Spacing between the Iteration label and value (ie: none) should be 8px/1rem