Refactor issues list page bulk edit from Haml to Vue

Background

The issues list page is being refactored from Haml to Vue in #322755 (closed) but does not include refactoring the bulk edit sidebar due to the complexity of the dropdowns. Once we have reusable widgets available for the assignees, milestones, etc dropdown, which is happening in &5302, then we can attempt to refactor the bulk edit sidebar as it should involve less effort due to these reusable widgets.

Technical information

• The Haml bulk edit sidebar is rendered in app/views/projects/issues/index.html.haml
• The JS logic for the bulk edit sidebar is primarily in app/assets/javascripts/issuable_bulk_update_actions.js and app/assets/javascripts/issuable_bulk_update_sidebar.js
• The Haml bulk edit sidebar was integrated with the Vue issues list in !57230 (merged). The bulk edit refactor should remove any code added to app/assets/javascripts/issuable_list/components/issuable_item.vue and app/assets/javascripts/issuable_list/components/issuable_list_root.vue in this MR to clean it up