Replace 'alias_method_chain :save_attachments, :pasted_images' in Rails 5.2.3

I've got a Redmine 3.4.4 installation running, which uses Ruby 2.2.5-p319 and Rails 4.2.8. We want to upgrade this to Redmine latest (currently 4.0.4) which requires Rails 5.

I'm running the new server using 4.0.4 using Ruby 2.6.3-p62 and Rails 5.2.3. Overall it's OK, but we have a number of plugins installed we would like to migrate. Many of these have had problems because of deprecations in Rails 5. I've managed to muddle my way through 8 of 9 plugins, even though I've not written Ruby on Rails before, but I'm stuck on the last one and just can't figure it out.

The plugin is the My Page Customization plugin and when I attempt to migrate the database and plugins I get this error:

[centos@redmine]$ bundle exec rake db:migrate RAILS_ENV=production
rake aborted!
NoMethodError: undefined method `alias_method_chain' for ActivitiesController:Class
Did you mean?  alias_method
/usr/local/src/redmine-4.0.4/plugins/redmine_my_page/lib/my_page_patches/activities_controller_patch.rb:11:in `block in included'
/usr/local/src/redmine-4.0.4/plugins/redmine_my_page/lib/my_page_patches/activities_controller_patch.rb:7:in `class_eval'
/usr/local/src/redmine-4.0.4/plugins/redmine_my_page/lib/my_page_patches/activities_controller_patch.rb:7:in `included'
/usr/local/src/redmine-4.0.4/plugins/redmine_my_page/init.rb:30:in `include'
/usr/local/src/redmine-4.0.4/plugins/redmine_my_page/init.rb:30:in `block (2 levels) in <top (required)>'

So, it's clear that the deprecated 'alias_method_chain' is the issue here. After some digging I found a lot of references online such as this one which is good and clear, but I just can't get code written that works - I keep getting syntax errors and can't figure out what I'm doing wrong.

This is the original snippet from activities_controller_patch.rb:

  module ActivitiesControllerPatch

    def self.included(base) # :nodoc:
      base.send(:include, InstanceMethods)

      base.class_eval do
        unloadable
        helper :issues
        helper :queries
        alias_method_chain :index, :esi
      end
    end

We'd like to hang onto this plugin if we can even though it doesn't officially support Redmine 4. I'm hoping someone with better Ruby knowledge will be able to help.

Instead of

alias_method_chain :index, :esi

you'll just use

alias_method :index_without_esi, :index
alias_method :index, :index_with_esi

It was some kind of syntactic sugar.