Location sharing via QR codes

[lentschi]

The web instance should be able to print a QR code for any location the logged in user is related to. After another user scans said code with the app, the location should be automatically shared with them.

If the same code is scanned by a different barcode scanner app that supports opening URLs, the web instance's root page should open. (Where links to the app at Google Play, Apple's AppStore and the Microsoft Store can be found.)