FEATURE: Let staff add custom post notices. (#7377)

app/controllers/posts_controller.rb

@@ -476,6 +476,22 @@ class PostsController < ApplicationController
     render_json_dump(locked: post.locked?)
   end
 
+  def notice
+    raise Discourse::NotFound unless guardian.is_staff?
+
+    post = find_post_from_params
+
+    if params[:notice].present?
+      post.custom_fields["notice_type"] = Post.notices[:custom]
+      post.custom_fields["notice_args"] = params[:notice]
+      post.save_custom_fields
+    else
+      post.delete_post_notices
+    end
+
+    render body: nil
+  end
+
   def bookmark
     if params[:bookmarked] == "true"
       post = find_post_from_params