Revamp the CI system to be consistent with org changes from the rebranding

[kvanbere]

Since the rebranding, some of the CI infrastructure has developed issues/regressions and no longer works correctly.

1. Review which parts of the CI infrastructure require fixing.
2. Look into the possibility of using a different provider (i.e. Appveyor for Linux or Circle CI).

[kvanbere]

The maturity of Haskell on the CI infrastructure has come a long way. GitHub actions supports Linux, OS X and Windows builds, so we can trivially simplify the CI architecture by moving everything to GitHub actions and cancelling all the other providers.