Add slack extension for recent changes

[amrav]

I used git's subtree merge feature to add kulttuuri/slack_mediawiki into our repo in a subdirectory. This lets us keep the code in a single repository under source control, make changes ourselves, and pick changes from upstream if we like them.

The first commit is just the subtree merge under mediawiki/extensions/SlackNotifications.

The second commit adds a new send method to the extension, called exec_curl, which forks a new curl process to post to Slack without blocking the edit request path. This is more expensive/slow than using a proper event loop (the way we were doing with Redis + Python previously), but at our request rate it doesn't matter, and is much simpler.

[nishnik]

Thanks @amrav for this. I would run it on my local machine by tonight and see if everything's alright. Meanwhile, we should add the Batman logo.

[amrav]

Thanks, added the logo to the slack webhook settings.

You're welcome to try it on your machine of course, but I don't think it's strictly necessary to be able to review a patch that the reviewer needs to run the code. It can help if you're unsure about whether/how something will work though.

[amrav]

That's correct, no error handling. Errors are logged though, so we can go and look at what went wrong later. If we find that errors frequently happen, we can look into adding a persistent queue, like Redis. Unfortunately php doesn't have built-in async support, and mediawiki doesn't use an event loop either.