Feat: Make Join Vote button fully functional

• Check whether a vote exists (if not, give error)
• Check whether vote is already running
• If not running, go to /vote
• If running, go to /watch
• If closed, go to /results

If already running or already closed, also display a notification about this.

Nice to have: While waiting for all the API calls to complete, display a loading spinner in the button (aria-busy="true")