Reuse code for assignee dropdown

Followup of discussion on !1916 (merged).

The FormHelper#issue_assignees_dropdown_options fills an hash of attributes to be used by the assignee dropdown. But on several places, this helper method is not used (completely)

• app/views/projects/boards/components/sidebar/_assignee.html.haml
• app/views/shared/issuable/_sidebar_assignees.html.haml

Better would be to use DropdownsHelper#dropdown_tag, just like app/views/shared/issuable/form/_metadata_issue_assignee.html.haml does. Maybe even put the assignee dropdown in a separate partial.

cc @ClemMakesApps