FormHelper#issue_assignees_dropdown_options knows if it has multiple assignees

Instead of telling the `FormHelper#issue_assignees_dropdown_options` if there
are multiple assignees enabled, make it figure it out itself.

The implementation for CE will never have multiple
assignees. `EE::FormHelper#issue_assignees_dropdown_options` will have it if the
feature is enabled in the license.

app/helpers/ee/form_helper.rb

+module EE
+  module FormHelper
+    def issue_assignees_dropdown_options
+      options = super
+
+      if @project.feature_available?(:multiple_issue_assignees)
+        options[:title] = 'Select assignee(s)'
+        options[:data][:'dropdown-header'] = 'Assignee(s)'
+        options[:data].delete(:'max-select')
+      end
+
+      options
+    end
+  end
+end

app/helpers/form_helper.rb

 module FormHelper
+  prepend ::EE::FormHelper
+
   def form_errors(model)
     return unless model.errors.any?
 
@@ -16,8 +18,8 @@ module FormHelper
     end
   end
 
-  def issue_dropdown_options(issuable, has_multiple_assignees = true)
-    options = {
+  def issue_assignees_dropdown_options
+    {
       toggle_class: 'js-user-search js-assignee-search js-multiselect js-save-user-data',
       title: 'Select assignee',
       filter: true,
@@ -27,8 +29,8 @@ module FormHelper
         first_user: current_user&.username,
         null_user: true,
         current_user: true,
-        project_id: issuable.project.try(:id),
-        field_name: "#{issuable.class.model_name.param_key}[assignee_ids][]",
+        project_id: @project.id,
+        field_name: "issue[assignee_ids][]",
         default_label: 'Unassigned',
         'max-select': 1,
         'dropdown-header': 'Assignee',
@@ -38,13 +40,5 @@ module FormHelper
         current_user_info: current_user.to_json(only: [:id, :name])
       }
     }
-
-    if has_multiple_assignees
-      options[:title] = 'Select assignee(s)'
-      options[:data][:'dropdown-header'] = 'Assignee(s)'
-      options[:data].delete(:'max-select')
-    end
-
-    options
   end
 end

app/views/shared/issuable/form/_metadata_issue_assignee.html.haml

@@ -7,5 +7,5 @@
     - if issuable.assignees.length === 0
       = hidden_field_tag "#{issuable.to_ability_name}[assignee_ids][]", 0, id: nil, data: { meta: '' }
 
-    = dropdown_tag(users_dropdown_label(issuable.assignees), options: issue_dropdown_options(issuable,true))
+    = dropdown_tag(users_dropdown_label(issuable.assignees), options: issue_assignees_dropdown_options)
   = link_to 'Assign to me', '#', class: "assign-to-me-link #{'hide' if issuable.assignees.include?(current_user)}"