Set up CI on github

[Vinai]

Probably github actions Need to run unit and integration tests on PHP 7.4 and PHP 7.3 for each branch/PR.

~Need to clean up integration tests first so they don't require sample data to be installed.~ 

• [x] Run Integration Tests on PHP 7.4 by default
• [x] Run Integration Tests on PHP 7.3 on php73 branch
• [x] Run Unit Tests on PHP 7.4 by default
• [x] Run Unit Tests on PHP 7.3 on php73 branch
• [ ] Run integration tests after rector.php backport on PHP 7.3 (except on php73 branch)
  • Use separate workflow so it can be easily disabled on php73 branch
  • Run unit and integration on the same job since the backport requires SCD and hence a full install anyway
  • Use MAGENTO_POST_INSTALL_SCRIPT for backporting

[Vinai]

Currently integration and unit tests are separate workflows. Reminder to self: check if combining integration and unit tests as separate jobs in a single workflow is better. Confirmed: combined is nicer.

[Vinai]

Can't install rector or rector-prefixed in Magento due to deps hell and can't use rector docker because I want it to run in an unprivileged docker github action. Don't want to spend the time to build a custom image and action.

Going to have to backport and test locally.