Migrate spec/javascripts/vue_shared/components/issue/ to Jest

The following test files should be refactored from Karma to Jest:

find spec/javascripts/vue_shared/components/issue -name '*_spec.js' -exec echo "- [ ] {}" \;

• spec/javascripts/vue_shared/components/issue/issue_warning_spec.js
• spec/javascripts/vue_shared/components/issue/issue_milestone_spec.js
• spec/javascripts/vue_shared/components/issue/issue_assignees_spec.js
• spec/javascripts/vue_shared/components/issue/related_issuable_item_spec.js