Make multi-device syncing possible

[raphaellueckl]

For instance, use chrome.storage which could be synced over the network (not the best solution since browser specific) or try anything with WebRTC.

[markus-walther]

This could perhaps be solved with https://workers.cloudflare.com.

Here is a relevant discussion: https://community.cloudflare.com/t/workers-server-sent-events-server-push/184219.

[markus-walther]

This was merged in a first implementation via PR #36 today. Some aspects warrant a follow-up PR for cleanup and polishing:

• minified QR-code scanner files make it hard to improve performance there, e.g. to avoid double scanning for normal and inverted QR codes, or fix the bad default algorithm for image binarization, or to finetune how many images are sent to the worker (if native accelerated decoding is not available, e.g. on iOS)
• debug UI for text sending and connection state should be removed
• call-to-action button texts should refer to sync task at hand
• QR code should be page-filling in size, i.e. may need to enlarge overlay
• WebRTC offer/answer JSONs should be compressed to only their variable parts for slimmer QR codes and uncompressed upon scanning
• video element only for scanning, QR-code only for generation - not both visible at the same time
• have a details element in the UI with instructions in German, to explain the sequence of user actions for first-time users
• fix Uncaught (in promise) DOMException: The play() request was interrupted by a new load request. https://goo.gl/LdLk22
• fix excessive CPU for long-running QR-code scanning (e.g. timeout for scanner-active)