Remove the use of issuable.assignees in sidebar

Follow up for gitlab-ce!23379 "Create a serializer to render issuables sidebar"

The sidebar is rendered using the issuable_sidebar Hash from the serializer except for the assignees section.

The issuable.assignees is still used even if the data is also available in the Hash because the helpers for rendering the assignee (with avatar and link) expects it to be a model and it was hard to change at that time.

Rendering this in Vue would eliminate the need for those helpers. For issues, they are already rendered in Vue because they are loaded async, so there should already be a component for that.

The other place it is used is to populate the selected values of the assignee select dropdown. This applies to both issues and merge requests. Even if issues.assignees are loaded async, they were still loaded synchronously here to populate the select. We need to do that async too and might be easier when this select box is also refactored to Vue so we don't need to create hidden input elements from Vue.