Greeter revamp: Hide "discover", "quick setup", and "Start" button when not relevant

From the feedback from experts (#8235 (closed)):

• Buttons at the bottom (discover, quick setup, start) are useless [no more buttons]

Attachments

• 
• Start.Tails.Button.Location.svgz

Parent Task: #8231 (closed)

Original created by @sajolida on 8971 (Redmine)