Meta: hook up CI into Slack for more of our projects?

[fingolfin]

Right now, the results of the Travis CI tests for GAP.jl are reported in a special channel on the oscar-system Slack. I find this quite useful because it allows me to quickly see if the build starts failing there for some reason. In addition, the tests there are run periodically every 24 hours, so I also quickly see when Julia nightly starts to cause issues.

Perhaps we can do the same for more of our subprojects, at least those using Travis. It's not that hard to set up, see

• https://docs.travis-ci.com/user/notifications/#configuring-slack-notifications and
• https://github.com/oscar-system/GAP.jl/blob/0ef9fb33415fafb0e19dd29393ef367668c335ef/.travis.yml#L79-L91

Ideally something similar would be done for GitHub actions as used by e.g. Polymake.jl. There are several tools on the GitHub market place which claim to allow this, e.g. these (found by a quick google search, there might be others and better ones):

• https://github.com/marketplace/actions/slack-notify
• https://github.com/marketplace/actions/github-action-for-slack