Add cooking mode

[dubadub]

A big one, probably makes sense to split into subissues.

Add cooking mode to WebUI:

• should be optimised for tablet experience
• non-sleeping mode
• big font, one step only shown at a time
• step shows required ingredients
• swipe left/right
• setting timer
• show step images

[JeppeKlitgaard]

Please also consider whether it might make sense for this to be optimised for phones as well