Web sockets: Split out into WebSockets.ino, support multiple connections

[LeeLeahy2]

This pull request includes the following commits:

• WebSockets: Split out webSockets from webServer
• WebSockets: Use webSocketsIsConnected() instead of websocketConnected
• WebSockets: Update start messages
• menuCommands: Protect against settingsCSV being nullptr
• WebSockets: Add webSockets to the messages
• webSockets: Add list of active clients

[PaulZC]

Hi Lee (@LeeLeahy2 ) and Nathan (@nseidle ),

This is good stuff!

But please be aware that there is a lot of overlap between this PR and #827.

I propose we merge this PR and #832 first. If there is anything from #827 that still needs to be included, I will open a new PR to cover that.

(If we try to merge #827 and this PR, we'll get into a big old mess!)

All the best,
Paul