Take Leaflet.markercluster from npm instead of rails-assets.org - #4836

Drops the project's last dependency on rails-assets.org, which is down.

Why

Bundler can't install anything while one of the Gemfile's sources is unreachable. With rails-assets.org down, every CI job fails at bundle install, on every branch, before a single spec runs ("Could not reach host rails-assets.org"). #4835 is failing this way now.

The only gem from there was rails-assets-leaflet.markercluster, the marker clustering used on the places and crop maps.

What changes

• The cluster plugin comes from npm. leaflet.markercluster is now an npm package. node_modules is already on the asset path, so application.js and application.scss load it from there. CI and Heroku both install npm packages already, via the yarn steps and jsbundling-rails.
• The maps move from Leaflet 1.5.1 to 1.9.4. The rails-assets gem also brought in rails-assets-leaflet, a second copy of Leaflet at 1.5.1, and that copy was the one being served, shadowing the leaflet-rails gem the Gemfile asks for (>= 1.9.2). With it gone, Leaflet comes from leaflet-rails, as the Gemfile intended, and there's one copy instead of two.
• The plugin moves from 1.4.1 to 1.5.3. That's its current release, and it supports Leaflet 1.9.

yarn warns that the plugin's leaflet peer dependency is missing. That's expected: Leaflet comes from the gem and is loaded before the plugin, and an npm copy would just duplicate it again.

Checked

• The lockfile. Only the rails-assets section is removed from Gemfile.lock. It was relocked locally, with no network needed.
• What's served. The compiled application.js has one Leaflet, 1.9.4, and L.MarkerClusterGroup. application.css has Leaflet's and the cluster plugin's styles, and Leaflet's images resolve through the asset pipeline.
• The maps, in a browser. A throwaway spec, not committed, checked that the member, crop and places maps each draw, with their markers or clusters, on Leaflet 1.9.4 with the plugin present.
• Existing specs. The member profile, crop page and places specs pass: 75 examples, 0 failures.

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