Style invite page

[rosano]

Inspired by the equivalent for:

• Signal https://signal.me/#p/+1234567890
• Telegram https://t.me/+1234567890
• WhatsApp https://api.whatsapp.com/send?phone=1234567890

and includes DeltaChat branding, background image tile, only one call to action on the page, more responsiveness for mobile devices.