Move /reassign slash command to EE subfolder

The /reassign slash command only is used in EE, so move it outside the shared
code to avoid merge conflicts.

app/models/concerns/issuable.rb

@@ -23,6 +23,8 @@ module Issuable
   IssuableMeta = Struct.new(:upvotes, :downvotes, :notes_count, :merge_requests_count)
 
   included do
+    prepend EE::Issuable
+
     cache_markdown_field :title, pipeline: :single_line
     cache_markdown_field :description, issuable_state_filter_enabled: true
 

app/services/ee/slash_commands/interpret_service.rb

+module EE
+  module SlashCommands
+    module InterpretService
+      include ::Gitlab::SlashCommands::Dsl
+
+      desc 'Change assignee(s)'
+      explanation do
+        'Change assignee(s)'
+      end
+      params '@user1 @user2'
+      condition do
+        issuable.allows_multiple_assignees? &&
+          issuable.persisted? &&
+          issuable.assignees.any? &&
+          current_user.can?(:"admin_#{issuable.to_ability_name}", project)
+      end
+      command :reassign do |unassign_param|
+        @updates[:assignee_ids] = extract_users(unassign_param).map(&:id)
+      end
+    end
+  end
+end

app/services/quick_actions/interpret_service.rb

@@ -150,21 +150,6 @@ module QuickActions
       end
     end
 
-    desc 'Change assignee(s)'
-    explanation do
-      'Change assignee(s)'
-    end
-    params '@user1 @user2'
-    condition do
-      issuable.allows_multiple_assignees? &&
-        issuable.persisted? &&
-        issuable.assignees.any? &&
-        current_user.can?(:"admin_#{issuable.to_ability_name}", project)
-    end
-    command :reassign do |unassign_param|
-      @updates[:assignee_ids] = extract_users(unassign_param).map(&:id)
-    end
-
     desc 'Set milestone'
     explanation do |milestone|
       "Sets the milestone to #{milestone.to_reference}." if milestone