[DO NOT MERGE] Use type=module for application.js

[AshGDS]

What / Why

• Use type=module for this application's JS file
• To coincide with https://github.com/alphagov/govuk_publishing_components/pull/4111
• Has [DO NOT MERGE] as we want the above PR live on static first before this is merged.
• We will combine es6-components.js again as a separate task unless you disagree with this
• To accurately test this you will need to use local static that is using a local version the govuk_publishing_components branch linked above. If you test without, this app's application.js seems to crash.
• Trello card: https://trello.com/c/Hoa45SMD/141-turn-off-javascript-in-legacy-browsers