Replace user_select in app/assets/javascripts/issues/list/components/issues_list_app.vue

We're in the process of migrating away all uses of Bootstrap. Whilst this is a simple task for things such as buttons and alerts, dropdowns are much more complex. Because of that, there will be no step-by-step instructions in this issue.

This dropdown can (very likely) be migrated to our user_select Vue component, for more info see the epic: &8531

File: app/assets/javascripts/issues/list/components/issues_list_app.vue

frontend implementation plan:

Looks like it's just the "assignee" dropdown in the "bulk edit sidebar" that will be impacted by this issue.

• Remove existing dropdown_tag from app/views/shared/issuable/_bulk_update_sidebar.html.haml and replace it with a div with class to later target, e.g. js-assignee-dropdown
• Remove all references to old UsersSelect in app/assets/javascripts/issues/list/components/issues_list_app.vue
• Mount the user_select Vue component when the sidebar is initalised at target class from before, from app/assets/javascripts/issuable/issuable_bulk_update_sidebar.js
• Propagate selections from user_select into a hidden input in the haml to be submitted along with the original form.
• Tests