GitHub actions refactor

[mnocon]

Things done:

1. The travis executable is now renamed to ci
   The usage stays as before:

vendor/bin/ci dependencies:link
vendor/bin/ci regression:run

Or:

bin/ci dependencies:link
bin/ci regression:run

depending on Composer bin-dir setting.
2) Unused scripts have been removed: update_docker and check_cs were not really used and are not needed going forward (moving to GA)
3) Regression Command has been adjusted:

• it now opens a PR in the repository chosen by user (ibexa/oss, ibexa/content, ibexa/experience, ibexa/commerce) and runs the tests that are defined there
  

4. Relying on the prepare_project_edition script to be delived by Composer scripts forced us to do composer install inside every package - it was done this way to limit the amount of logic inside each Travis configuration file.
   With Github Actions we can do it differentlly, recipes style - each script will be kept in a separate directory and will be downloaded by the Github Actions job when needed. Thanks to it we don't have to perform composer install inside packages anymore to run browser tests. This also means that we won't be updating the 0.1 branch once we fully leave Travis behind.

[mnocon]

This file has been copied from https://github.com/ibexa/ci-scripts/blob/0.1/bin/prepare_project_edition.sh

[micszo]

Tested locally and works fine. 🎉

Thanks!