[FEATURE] Expose option to disable header to deployment config

[jfmcquade]

What?
Deployments should be able to disable the header.

Why?
Designs for the Kuwait app (see figma) include no header.

How?
The header should be able to be disabled generally at the deployment config level. We need to consider how to handle some of the functionality that is currently enabled by the header:

• Navigation
  • Critically, we need to ensure that removing the permanent "back" button doesn't compromise the ability of users to navigate within the app
  • This will likely involve exposing a nav_back action to authors and relying on templates being authored to include back buttons where necessary

Other details
Previous issue (usurped by alternative implementation): #2154