Implement EZP-20039: Automate install steps in composer post install/update

Todo:
- [x] Sync existing files (including index_dev protection)
- [x] Add SensioDistributionBundle, the composer install/update commands and use boostrap file in index.php
- [x] Add requirement checker (not including the generated files)
- [x] Add unit test dist file for running src Bundle unit tests
- [x] Add eZ Publish script execution in composer steps ( legacy assets, ??)
- [x] Technical doc
- [x] Merge [380](https://github.com/ezsystems/ezpublish-kernel/pull/380) and update composer.json to point to master again
- [x] Fix use of SensioDistributionBundle in Core to work even though the bundle is not active [#412](https://github.com/ezsystems/ezpublish-kernel/pull/412)

See issue for more info: https://jira.ez.no/browse/EZP-20039

Optional followup tasks:
- Update to Sf 2.3 and add Incenteev ParameterHandler + move parameters.yml to parameters.yml.dist
- Consider adding SensioFrameworkExtraBundle to solve [EZP-20398](https://jira.ez.no/browse/EZP-20398) (But there should be a performance audit in adding more bundles, including this one)