Automatically build and deploy adhoc builds

[turt2live]

Like the spec does

[turt2live]

So we can plug all the riot artifacts into Buildkite without paying for it, but Riot won't load off the CI unlike the spec (the spec uses CircleCI anyways). Buildkite uses a different directory/file name in their private s3 bucket for every file, so Riot can't use relative pathing to location it all.

One option might be to still abuse Buildkite's archiving and have @lampholder's riots.im/adhoc support a build number to download & extract a tgz of riot. In theory we could still stick a buildkite job on the project to report a status check to github with the URL, and at that point we're probably better off poking either riots.im/adhoc to just extract it then and there, or building our own NV-hosted version.

In any case, this now needs ops to figure out a good place to shove ~25mb worth of stuff for every build and get it hosted on a webserver somewhere. Note: this is not safe code to deploy to riot.im/develop - this would be PR builds from external and internal contributors.

[jryans]

Experimental preview builds are now enabled for future React SDK PRs! 🚀 At the moment, only the React SDK repo is covered. To access the preview site, look for netlify/matrix-react-sdk/deploy-preview in the checks and click "details" to go to the preview copy of Element.

For any problems or further enhancements, let's use new issues to track them.