Install importmap

- ran command `./bin/rails importmap:install`
- the result is probably wrong for our config, but keeping this in a separate commit for clarity before reconfiguring
- critically it failed to add anything to the application layout file, which we probably need to address in the layout component

Author: andysellick

app/assets/config/manifest.js

@@ -13,3 +13,5 @@
 //= link static-error-pages.js
 
 //= link_tree ../builds
+//= link_tree ../../javascript .js
+//= link_tree ../../../vendor/javascript .js

app/javascript/application.js

@@ -0,0 +1 @@
+// Configure your import map in config/importmap.rb. Read more: https://github.com/rails/importmap-rails

bin/importmap

@@ -0,0 +1,4 @@
+#!/usr/bin/env ruby
+
+require_relative "../config/application"
+require "importmap/commands"

config/importmap.rb

@@ -0,0 +1,3 @@
+# Pin npm packages by running ./bin/importmap
+
+pin "application"