Migrate to gitlab.nic.cz and keep a mirror on GitHub

[jiri-janousek]

Travis-ci.org is going to close soon forcing us to migrate to travis-ci.com. However, there are limited free credits on travis-ci.com, so we would need to pay jobs above the limit. I think it's better to use our infrastructure instead.

• django-eidas-specific-node project on our GitLab instance will remain public. Contributors are welcome.
• Repository will be mirrored on GitHub.
• We need to set up CI for third-party contributors. I doubt it will work out of the box.

We could perhaps switch to Circle CI or GitHub actions, but we are far more familiar with GitLab CI.