Install Node.js on deploy

We use FNM instead of NVM. Reason: the setup seems easier with just
`eval "$(fnm env)"`.

So now, we try to install Node.js; if the command fails, it could be
because FNM isn't installed (and we need to install it) or because the
version of Node.js cannot be installed with the current version of FNM
(and we need to update FNM). After installing/updating FNM, we try to
install Node.js again.

Note we're using `fnm env` in the middle of the `fnm_setup_command`.
That way, the command will raise a `SSHKit::Command::Failed` exception
if `fnm` isn't installed, and it will give us an indicator that we need
to actually install it.

config/deploy.rb

@@ -37,6 +37,16 @@ set :keep_releases, 5
 
 set :local_user, ENV["USER"]
 
+set :fnm_path, "$HOME/.fnm"
+set :fnm_install_command, "curl -fsSL https://fnm.vercel.app/install | " \
+                          "bash -s -- --install-dir \"#{fetch(:fnm_path)}\""
+set :fnm_update_command, "#{fetch(:fnm_install_command)} --skip-shell"
+set :fnm_setup_command, -> do
+                          "export PATH=\"#{fetch(:fnm_path)}:$PATH\" && " \
+                            "cd #{release_path} && fnm env > /dev/null && eval \"$(fnm env)\""
+                        end
+set :fnm_install_node_command, -> { "#{fetch(:fnm_setup_command)} && fnm use --install-if-missing" }
+
 set :puma_conf, "#{release_path}/config/puma/#{fetch(:rails_env)}.rb"
 
 set :delayed_job_workers, 2
@@ -49,6 +59,7 @@ namespace :deploy do
   Rake::Task["delayed_job:default"].clear_actions
   Rake::Task["puma:smart_restart"].clear_actions
 
+  after :updating, "install_node"
   after :updating, "install_ruby"
 
   after "deploy:migrate", "add_new_settings"
@@ -89,6 +100,26 @@ task :install_ruby do
   end
 end
 
+task :install_node do
+  on roles(:app) do
+    with rails_env: fetch(:rails_env) do
+      begin
+        execute fetch(:fnm_install_node_command)
+      rescue SSHKit::Command::Failed
+        begin
+          execute fetch(:fnm_setup_command)
+        rescue SSHKit::Command::Failed
+          execute fetch(:fnm_install_command)
+        else
+          execute fetch(:fnm_update_command)
+        end
+
+        execute fetch(:fnm_install_node_command)
+      end
+    end
+  end
+end
+
 task :refresh_sitemap do
   on roles(:app) do
     within release_path do