Make the Buttonweavers Discord server available to general Buttonweavers players

[blackshadowshade]

Related to #255.

We're now more than a decade on from initially wanting real-time chat, and technology has moved on too. Chaos and I have chatted about how to do real-time chat, and we've settled on a solution that has worked well amongst the developer team: Discord.

The idea would be to widen access to the Buttonweavers Discord server, which is currently only open to developers. Certain parts of the Discord would still be access-locked for developers only, but most of the server would then be available for Buttonweavers members.

[blackshadowshade]

The Discord server has been set up and is ready to go.

At some stage, we'll probably want to add the Discord link to the logged-in help pages, and then update the code of conduct to reflect that.