Now that Github CI exists, and Travis CI appears to have stopped functioning, move the automated testing over to use Github CI

Basic support added in #650. Need to enable more of the tests (currently commented out in the workflow config)

Remaining things to do:

• Remove old travis scripts.
• Update CI workflow to upload build artifacts.
• Enable more tests in CI workflow
• Get Windows build for C++ client working again (see #681)
• Split the workflow into multiple jobs (gh can run up to 20 jobs in parallel, so would speed up the workflow considerably)

Closing as this has been split into multiple issues so we can work on things in parallel