Use FormHelper#issue_assignees_dropdown_options for Issue sidebar

Avoid code duplication and limit the number of CE -> EE merge conflict by
reusing `FormHelper#issue_assignees_dropdown_options` to set some assignee
dropdown attributes.

app/views/projects/boards/components/sidebar/_assignee.html.haml

@@ -19,10 +19,11 @@
         ":data-name" => "assignee.name",
         ":data-username" => "assignee.username" }
       .dropdown
-        %button.dropdown-menu-toggle.js-user-search.js-author-search.js-multiselect.js-save-user-data.js-issue-board-sidebar{ type: "button", ref: "assigneeDropdown", data: { toggle: "dropdown", field_name: "issue[assignee_ids][]", first_user: (current_user.username if current_user), current_user: "true", project_id: @project.id, null_user: "true", multi_select: "true", 'max-select' => 1, dropdown: { header: 'Assignee' } },
+        - dropdown_options = issue_assignees_dropdown_options
+        %button.dropdown-menu-toggle.js-user-search.js-author-search.js-multiselect.js-save-user-data.js-issue-board-sidebar{ type: 'button', ref: 'assigneeDropdown', data: { toggle: 'dropdown', field_name: 'issue[assignee_ids][]', first_user: current_user&.username, current_user: 'true', project_id: @project.id, null_user: 'true', multi_select: 'true', 'dropdown-header': dropdown_options[:data][:'dropdown-header'], 'max-select': dropdown_options[:data][:'max-select'] },
           ":data-issuable-id" => "issue.id",
           ":data-issue-update" => "'#{namespace_project_issues_path(@project.namespace, @project)}/' + issue.id + '.json'" }
-          Select assignee
+          = dropdown_options[:title]
           = icon("chevron-down")
         .dropdown-menu.dropdown-select.dropdown-menu-user.dropdown-menu-selectable.dropdown-menu-author
           = dropdown_title("Assign to")

app/views/shared/issuable/_sidebar_assignees.html.haml

@@ -34,19 +34,20 @@
   - issuable.assignees.each do |assignee|
     = hidden_field_tag "#{issuable.to_ability_name}[assignee_ids][]", assignee.id, id: nil, data: { avatar_url: assignee.avatar_url, name: assignee.name, username: assignee.username }
 
-  - options = { toggle_class: 'js-user-search js-author-search', title: 'Assign to', filter: true, dropdown_class: 'dropdown-menu-user dropdown-menu-selectable dropdown-menu-author', placeholder: 'Search users', data: { first_user: (current_user.username if current_user), current_user: true, project_id: (@project.id if @project), author_id: issuable.author_id, field_name: "#{issuable.to_ability_name}[assignee_ids][]", issue_update: issuable_json_path(issuable), ability_name: issuable.to_ability_name, null_user: true } }
-
+  - options = { toggle_class: 'js-user-search js-author-search', title: 'Assign to', filter: true, dropdown_class: 'dropdown-menu-user dropdown-menu-selectable dropdown-menu-author', placeholder: 'Search users', data: { first_user: current_user&.username, current_user: true, project_id: @project&.id, author_id: issuable.author_id, field_name: "#{issuable.to_ability_name}[assignee_ids][]", issue_update: issuable_json_path(issuable), ability_name: issuable.to_ability_name, null_user: true } }
   - title = 'Select assignee'
 
   - if issuable.is_a?(Issue)
     - unless issuable.assignees.any?
       = hidden_field_tag "#{issuable.to_ability_name}[assignee_ids][]", 0, id: nil
+    - dropdown_options = issue_assignees_dropdown_options
+    - title = dropdown_options[:title]
     - options[:toggle_class] += ' js-multiselect js-save-user-data'
     - data = { field_name: "#{issuable.to_ability_name}[assignee_ids][]" }
     - data[:multi_select] = true
     - data['dropdown-title'] = title
-    - data['dropdown-header'] = 'Assignee'
-    - data['max-select'] = 1
+    - data['dropdown-header'] = dropdown_options[:data][:'dropdown-header']
+    - data['max-select'] = dropdown_options[:data][:'max-select']
     - options[:data].merge!(data)
 
   = dropdown_tag(title, options: options)