don't prefer polling

[jkeat]

Description

• Don't prefer polling for waitlist
• the version of vue-socket.io we have requires a 1.x version of socket.io-client, which is apparently mostly compatible with the 2.x server-side socket.io we have, besides

it only (partly) breaks polling transport, so you can mix 1.x and 2.0 versions with websocket transport (which is the protocol used in the vast majority of cases)

• 2.x and 1.x compatibility socketio/socket.io#3007 (comment)
• The versioning issues are addressed in these PRs
  • Fault tolerance #413
  • https://github.com/UPchieve/server/pull/351

Developer self-review checklist

• Potentially confusing code has been explained with comments
• No warnings or errors have been introduced; all known error cases have been handled
• Any appropriate documentation (within the code, README.md, docs, etc) has been updated
• All edge cases have been addressed

[treystevens]

Looks good!