Since Foundation hasn't released a new gem for years, we haven't been
able to upgrade Foundation to its most recent version.
Thanks to this change, we'll be able to do so.
Note we're using motion-ui version 2.0.3 because version 2.0.5 (the
latest at the moment) requires Dart Sass.
Other packages depend on jQuery, so that's why these are the first one
we move from the Gemfile to the package.json file.
This way we can also test whether dependabot correctly opens pull
requests to update Node packages.
I've tried several configuration options for the asset pipeline in order
to be able to include images referenced in jQuery UI CSS files. So far,
adding the `node_modules/jquery-ui/themes/base` folder to the assets
paths is the only way I've found to make it work. Hopefully we can find
a better solution in the future so we don't have to study the internals
of every Node package in order to integrate it with the assets pipeline.
Note that, even if we're excluding the `node_modules/` folder from
version control, we aren't adding it to Capistrano's shared folders
because, when `node_modules` is a symbolic link, NPM removes it when
running `npm install`.
