Upgrading RailsAdmin from 2.x.x to 3.x.x

[vczb]

When executing bundle install command this message appear on the console:

  Post-install message from rails_admin:

    ### Upgrading RailsAdmin from 2.x.x to 3.x.x ###

    Due to introduction of Webpack/Webpacker support, some additional dependencies and configuration will be needed.
    Running `bin/rails g rails_admin:install` will suggest required changes, based on the current setup of your app.

    For a complete list of changes, see https://github.com/railsadminteam/rails_admin/blob/master/CHANGELOG.md

you need to figure out what need to be done and submit a PR