Discord chat bot to announce projects

[smartin015]

Closes #589

This adds support for a Discord bot that announces when interesting things happen on the backend. It's currently set up to notify when someone saves a new project image, as a proxy for "done-ness" to the point where folks might want to take a look and comment on the project. It has some limitations to keep the usage simple - it only sends text, and only to a single channel configured on startup (see .env.defaults). However, the bot (and lib) could very easily be modified to do more advanced stuff, e.g. responding to user chats from Discord.

While I was able to successfully set up a bot and have it talk on a channel, this PR isn't quite complete:

• I wasn't able to fully test the actual notifications as I'm missing CLOUDINARY_API_SECRET in my env - it prevents me from setting a project image and thus firing the bot message when the project is updated.
• IMO the chat message should be a URL to the updated project (easy to click and follow) but I'm not quite sure how to generate a link to the project from the backend. Any thoughts on how to do this in a nice way?

[Irev-Dev]

This looks really promising 🚀 , I'll respond to you properly after work.

[Irev-Dev]

I made a PR to this PR haha, https://github.com/smartin015/cadhub/pull/1 (I don't like stomping on other people's branches with my commits)

I think this is ready if you are happy with the changes I made, here's what the messages look like with the image.

[smartin015]

Yeah, this looks great! Ready to merge when you are.