[FEATURE] Option to remove app header

[esmeetewinkel]

What?
Ability to create a header-less app, see Figma mockups.

N.B. @jfmcquade I'm not sure whether to interpret the proposed navigation as a header-less app, or a "minimal header" together with a load of full-screen pop-ups.

Why?
Less cluttered design, more screen space to display content.

How?
Find below some notes I took when we discussed this in the dev catchup on 19 Oct 2023 (@jfmcquade please replace with more understandable notes if you have any!)

1. Menu variable in config to hide header
2. If we want a non-sticky (floating) header, proposal to have named layouts, e.g.
   • Header + sidebar
   • Header only
   • Non-sticky header
   • No header
   • Minimal header (only back button, no background colour)
3. Knock-ons for navigation:
   • Header back button
   • iOS navigation - has swipe back action so could confuse with side menu or interacting with a slider component
   • Navigation not custom applied to each template