Feat: Enable vote sharing

A vote should be able to be shared with a link, QR Code and the raw ID. Optionally we can also add a sharing prompt for mobile devices, not sure how much value there is to that over a simple link that can be copied (maybe also add a "copy link to clipboard" button).

For that we need the vote ID from the backend.

The vote URL can look like this: ${window.location}/vote?id=${voteId}

So in total: https://vote.fsr.imn.htwk-leipzig.de/vote?id=1234 (base URL is still to be decided, just a placeholder for now)