Allow joining documents via QR code

[bertob]

This has two components:

1. Showing a QR code to share the document (useful independently of the second point since you can scan with e.g. Decoder)
2. Scanning a QR code to join a document

The former is probably easier than the latter, and would be useful independently. The latter is mostly useful on mobile, but not exclusively (it's not very ergonomic, but doable to scan QR codes on laptops).

Mockup for sharing:

Mockup for scanning: