Speed up CI builds

[sergei-maertens]

Possible optimizations:

• [ ] Tag migration tests and run them in a separate job (set up a matrix for the tests job) - migration tests take 144s on my machine (that's 2.5 minutes!)
• [ ] Skip the webpack build for backend tests if possible (build takes 50s, installing deps takes another 20s -> could save a minute)
• [ ] Update to latest digid-eherkenning/lxml/xmlsec versions which can install binary wheels again instead of compiling it ourselves
• [ ] Replace Camunda docker-compose in CI with VCR cassettes