Add new multiple routes to large admin post list

[thomasiles]

Add new multiple routes to large admin post list

Trello card: https://trello.com/c/kdSpmSY2/3094-allow-larger-post-request-to-multiple-routes-page

The new page for multiple routes requires submitting a larger POST body.

The new page scales based on the number of pages and the number selection options in the form. The full design isn't finalised yet, and the feature is currently behind a feature flag.

Allowing the page to use larger POST bodies now will allow us to test the feature with larger forms.

I followed the pattern for the Welsh page. I've tested this on dev and it worked!

Things to consider when reviewing

• Ensure that you consider the wider context.
• Does it work when run on your machine?
• Is it clear what the code is doing?
• Do the commit messages explain why the changes were made?
• Are there all the unit tests needed?
• Has all relevant documentation been updated?

Reminders

If you've made changes to the deployer role (files in modules/deployer-access):

• Remember to run make <environment> forms/account apply on the relevant environments (dev, staging and/or prod)
• Check the #govuk-forms-deployment-notifications Slack channel to ensure the apply-forms-terraform-<environment> pipelines have run successfully